vendor/chrono-0.4.19/README.md - toolchain/rustc - Git at Google

 [Chrono][docsrs]: Date and Time for Rust
 ========================================

 [![Chrono GitHub Actions][gh-image]][gh-checks]
 [![Chrono on crates.io][cratesio-image]][cratesio]
 [![Chrono on docs.rs][docsrs-image]][docsrs]
 [![Join the chat at https://gitter.im/chrono-rs/chrono][gitter-image]][gitter]

 [gh-image]: https://github.com/chronotope/chrono/workflows/test/badge.svg
 [gh-checks]: https://github.com/chronotope/chrono/actions?query=workflow%3Atest
 [cratesio-image]: https://img.shields.io/crates/v/chrono.svg
 [cratesio]: https://crates.io/crates/chrono
 [docsrs-image]: https://docs.rs/chrono/badge.svg
 [docsrs]: https://docs.rs/chrono
 [gitter-image]: https://badges.gitter.im/chrono-rs/chrono.svg
 [gitter]: https://gitter.im/chrono-rs/chrono

 It aims to be a feature-complete superset of
 the [time](https://github.com/rust-lang-deprecated/time) library.
 In particular,

 * Chrono strictly adheres to ISO 8601.
 * Chrono is timezone-aware by default, with separate timezone-naive types.
 * Chrono is space-optimal and (while not being the primary goal) reasonably efficient.

 There were several previous attempts to bring a good date and time library to Rust,
 which Chrono builds upon and should acknowledge:

 * [Initial research on
    the wiki](https://github.com/rust-lang/rust-wiki-backup/blob/master/Lib-datetime.md)
 * Dietrich Epp's [datetime-rs](https://github.com/depp/datetime-rs)
 * Luis de Bethencourt's [rust-datetime](https://github.com/luisbg/rust-datetime)

 Any significant changes to Chrono are documented in
 the [`CHANGELOG.md`](https://github.com/chronotope/chrono/blob/main/CHANGELOG.md) file.


 ## Usage

 Put this in your `Cargo.toml`:

 ```toml
 [dependencies]
 chrono = "0.4"
 ```

 ### Features

 Chrono supports various runtime environments and operating systems, and has
 several features that may be enabled or disabled.

 Default features:

 - `alloc`: Enable features that depend on allocation (primarily string formatting)
 - `std`: Enables functionality that depends on the standard library. This
   is a superset of `alloc` and adds interoperation with standard library types
   and traits.
 - `clock`: enables reading the system time (`now`), independent of whether
   `std::time::SystemTime` is present, depends on having a libc.

 Optional features:

 - `wasmbind`: Enable integration with [wasm-bindgen][] and its `js-sys` project
 - [`serde`][]: Enable serialization/deserialization via serde.
 - `unstable-locales`: Enable localization. This adds various methods with a
   `_localized` suffix. The implementation and API may change or even be
   removed in a patch release. Feedback welcome.

 [`serde`]: https://github.com/serde-rs/serde
 [wasm-bindgen]: https://github.com/rustwasm/wasm-bindgen

 See the [cargo docs][] for examples of specifying features.

 [cargo docs]: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#choosing-features

 ## Overview

 ### Duration

 Chrono currently uses its own [`Duration`] type to represent the magnitude
 of a time span. Since this has the same name as the newer, standard type for
 duration, the reference will refer this type as `OldDuration`.

 Note that this is an "accurate" duration represented as seconds and
 nanoseconds and does not represent "nominal" components such as days or
 months.

 When the `oldtime` feature is enabled, [`Duration`] is an alias for the
 [`time::Duration`](https://docs.rs/time/0.1.40/time/struct.Duration.html)
 type from v0.1 of the time crate. time v0.1 is deprecated, so new code
 should disable the `oldtime` feature and use the `chrono::Duration` type
 instead. The `oldtime` feature is enabled by default for backwards
 compatibility, but future versions of Chrono are likely to remove the
 feature entirely.

 Chrono does not yet natively support
 the standard [`Duration`](https://doc.rust-lang.org/std/time/struct.Duration.html) type,
 but it will be supported in the future.
 Meanwhile you can convert between two types with
 [`Duration::from_std`](https://docs.rs/time/0.1.40/time/struct.Duration.html#method.from_std)
 and
 [`Duration::to_std`](https://docs.rs/time/0.1.40/time/struct.Duration.html#method.to_std)
 methods.

 ### Date and Time

 Chrono provides a
 [**`DateTime`**](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html)
 type to represent a date and a time in a timezone.

 For more abstract moment-in-time tracking such as internal timekeeping
 that is unconcerned with timezones, consider
 [`time::SystemTime`](https://doc.rust-lang.org/std/time/struct.SystemTime.html),
 which tracks your system clock, or
 [`time::Instant`](https://doc.rust-lang.org/std/time/struct.Instant.html), which
 is an opaque but monotonically-increasing representation of a moment in time.

 `DateTime` is timezone-aware and must be constructed from
 the [**`TimeZone`**](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html) object,
 which defines how the local date is converted to and back from the UTC date.
 There are three well-known `TimeZone` implementations:

 * [**`Utc`**](https://docs.rs/chrono/0.4/chrono/offset/struct.Utc.html) specifies the UTC time zone. It is most efficient.

 * [**`Local`**](https://docs.rs/chrono/0.4/chrono/offset/struct.Local.html) specifies the system local time zone.

 * [**`FixedOffset`**](https://docs.rs/chrono/0.4/chrono/offset/struct.FixedOffset.html) specifies
   an arbitrary, fixed time zone such as UTC+09:00 or UTC-10:30.
   This often results from the parsed textual date and time.
   Since it stores the most information and does not depend on the system environment,
   you would want to normalize other `TimeZone`s into this type.

 `DateTime`s with different `TimeZone` types are distinct and do not mix,
 but can be converted to each other using
 the [`DateTime::with_timezone`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.with_timezone) method.

 You can get the current date and time in the UTC time zone
 ([`Utc::now()`](https://docs.rs/chrono/0.4/chrono/offset/struct.Utc.html#method.now))
 or in the local time zone
 ([`Local::now()`](https://docs.rs/chrono/0.4/chrono/offset/struct.Local.html#method.now)).

 ```rust
 use chrono::prelude::*;

 let utc: DateTime<Utc> = Utc::now();       // e.g. `2014-11-28T12:45:59.324310806Z`
 let local: DateTime<Local> = Local::now(); // e.g. `2014-11-28T21:45:59.324310806+09:00`
 ```

 Alternatively, you can create your own date and time.
 This is a bit verbose due to Rust's lack of function and method overloading,
 but in turn we get a rich combination of initialization methods.

 ```rust
 use chrono::prelude::*;
 use chrono::offset::LocalResult;

 let dt = Utc.ymd(2014, 7, 8).and_hms(9, 10, 11); // `2014-07-08T09:10:11Z`
 // July 8 is 188th day of the year 2014 (`o` for "ordinal")
 assert_eq!(dt, Utc.yo(2014, 189).and_hms(9, 10, 11));
 // July 8 is Tuesday in ISO week 28 of the year 2014.
 assert_eq!(dt, Utc.isoywd(2014, 28, Weekday::Tue).and_hms(9, 10, 11));

 let dt = Utc.ymd(2014, 7, 8).and_hms_milli(9, 10, 11, 12); // `2014-07-08T09:10:11.012Z`
 assert_eq!(dt, Utc.ymd(2014, 7, 8).and_hms_micro(9, 10, 11, 12_000));
 assert_eq!(dt, Utc.ymd(2014, 7, 8).and_hms_nano(9, 10, 11, 12_000_000));

 // dynamic verification
 assert_eq!(Utc.ymd_opt(2014, 7, 8).and_hms_opt(21, 15, 33),
            LocalResult::Single(Utc.ymd(2014, 7, 8).and_hms(21, 15, 33)));
 assert_eq!(Utc.ymd_opt(2014, 7, 8).and_hms_opt(80, 15, 33), LocalResult::None);
 assert_eq!(Utc.ymd_opt(2014, 7, 38).and_hms_opt(21, 15, 33), LocalResult::None);

 // other time zone objects can be used to construct a local datetime.
 // obviously, `local_dt` is normally different from `dt`, but `fixed_dt` should be identical.
 let local_dt = Local.ymd(2014, 7, 8).and_hms_milli(9, 10, 11, 12);
 let fixed_dt = FixedOffset::east(9 * 3600).ymd(2014, 7, 8).and_hms_milli(18, 10, 11, 12);
 assert_eq!(dt, fixed_dt);
 ```

 Various properties are available to the date and time, and can be altered individually.
 Most of them are defined in the traits [`Datelike`](https://docs.rs/chrono/0.4/chrono/trait.Datelike.html) and
 [`Timelike`](https://docs.rs/chrono/0.4/chrono/trait.Timelike.html) which you should `use` before.
 Addition and subtraction is also supported.
 The following illustrates most supported operations to the date and time:

 ```rust

 use chrono::prelude::*;
 use chrono::Duration;

 // assume this returned `2014-11-28T21:45:59.324310806+09:00`:
 let dt = FixedOffset::east(9*3600).ymd(2014, 11, 28).and_hms_nano(21, 45, 59, 324310806);

 // property accessors
 assert_eq!((dt.year(), dt.month(), dt.day()), (2014, 11, 28));
 assert_eq!((dt.month0(), dt.day0()), (10, 27)); // for unfortunate souls
 assert_eq!((dt.hour(), dt.minute(), dt.second()), (21, 45, 59));
 assert_eq!(dt.weekday(), Weekday::Fri);
 assert_eq!(dt.weekday().number_from_monday(), 5); // Mon=1, ..., Sun=7
 assert_eq!(dt.ordinal(), 332); // the day of year
 assert_eq!(dt.num_days_from_ce(), 735565); // the number of days from and including Jan 1, 1

 // time zone accessor and manipulation
 assert_eq!(dt.offset().fix().local_minus_utc(), 9 * 3600);
 assert_eq!(dt.timezone(), FixedOffset::east(9 * 3600));
 assert_eq!(dt.with_timezone(&Utc), Utc.ymd(2014, 11, 28).and_hms_nano(12, 45, 59, 324310806));

 // a sample of property manipulations (validates dynamically)
 assert_eq!(dt.with_day(29).unwrap().weekday(), Weekday::Sat); // 2014-11-29 is Saturday
 assert_eq!(dt.with_day(32), None);
 assert_eq!(dt.with_year(-300).unwrap().num_days_from_ce(), -109606); // November 29, 301 BCE

 // arithmetic operations
 let dt1 = Utc.ymd(2014, 11, 14).and_hms(8, 9, 10);
 let dt2 = Utc.ymd(2014, 11, 14).and_hms(10, 9, 8);
 assert_eq!(dt1.signed_duration_since(dt2), Duration::seconds(-2 * 3600 + 2));
 assert_eq!(dt2.signed_duration_since(dt1), Duration::seconds(2 * 3600 - 2));
 assert_eq!(Utc.ymd(1970, 1, 1).and_hms(0, 0, 0) + Duration::seconds(1_000_000_000),
            Utc.ymd(2001, 9, 9).and_hms(1, 46, 40));
 assert_eq!(Utc.ymd(1970, 1, 1).and_hms(0, 0, 0) - Duration::seconds(1_000_000_000),
            Utc.ymd(1938, 4, 24).and_hms(22, 13, 20));
 ```

 ### Formatting and Parsing

 Formatting is done via the [`format`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.format) method,
 which format is equivalent to the familiar `strftime` format.

 See [`format::strftime`](https://docs.rs/chrono/0.4/chrono/format/strftime/index.html#specifiers)
 documentation for full syntax and list of specifiers.

 The default `to_string` method and `{:?}` specifier also give a reasonable representation.
 Chrono also provides [`to_rfc2822`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.to_rfc2822) and
 [`to_rfc3339`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.to_rfc3339) methods
 for well-known formats.

 Chrono now also provides date formatting in almost any language without the
 help of an additional C library. This functionality is under the feature
 `unstable-locales`:

 ```text
 chrono { version = "0.4", features = ["unstable-locales"]
 ```

 The `unstable-locales` feature requires and implies at least the `alloc` feature.

 ```rust
 use chrono::prelude::*;

 let dt = Utc.ymd(2014, 11, 28).and_hms(12, 0, 9);
 assert_eq!(dt.format("%Y-%m-%d %H:%M:%S").to_string(), "2014-11-28 12:00:09");
 assert_eq!(dt.format("%a %b %e %T %Y").to_string(), "Fri Nov 28 12:00:09 2014");
 assert_eq!(dt.format_localized("%A %e %B %Y, %T", Locale::fr_BE).to_string(), "vendredi 28 novembre 2014, 12:00:09");
 assert_eq!(dt.format("%a %b %e %T %Y").to_string(), dt.format("%c").to_string());

 assert_eq!(dt.to_string(), "2014-11-28 12:00:09 UTC");
 assert_eq!(dt.to_rfc2822(), "Fri, 28 Nov 2014 12:00:09 +0000");
 assert_eq!(dt.to_rfc3339(), "2014-11-28T12:00:09+00:00");
 assert_eq!(format!("{:?}", dt), "2014-11-28T12:00:09Z");

 // Note that milli/nanoseconds are only printed if they are non-zero
 let dt_nano = Utc.ymd(2014, 11, 28).and_hms_nano(12, 0, 9, 1);
 assert_eq!(format!("{:?}", dt_nano), "2014-11-28T12:00:09.000000001Z");
 ```

 Parsing can be done with three methods:

 1. The standard [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) trait
    (and [`parse`](https://doc.rust-lang.org/std/primitive.str.html#method.parse) method
    on a string) can be used for parsing `DateTime<FixedOffset>`, `DateTime<Utc>` and
    `DateTime<Local>` values. This parses what the `{:?}`
    ([`std::fmt::Debug`](https://doc.rust-lang.org/std/fmt/trait.Debug.html))
    format specifier prints, and requires the offset to be present.

 2. [`DateTime::parse_from_str`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_str) parses
    a date and time with offsets and returns `DateTime<FixedOffset>`.
    This should be used when the offset is a part of input and the caller cannot guess that.
    It *cannot* be used when the offset can be missing.
    [`DateTime::parse_from_rfc2822`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_rfc2822)
    and
    [`DateTime::parse_from_rfc3339`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_rfc3339)
    are similar but for well-known formats.

 3. [`Offset::datetime_from_str`](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html#method.datetime_from_str) is
    similar but returns `DateTime` of given offset.
    When the explicit offset is missing from the input, it simply uses given offset.
    It issues an error when the input contains an explicit offset different
    from the current offset.

 More detailed control over the parsing process is available via
 [`format`](https://docs.rs/chrono/0.4/chrono/format/index.html) module.

 ```rust
 use chrono::prelude::*;

 let dt = Utc.ymd(2014, 11, 28).and_hms(12, 0, 9);
 let fixed_dt = dt.with_timezone(&FixedOffset::east(9*3600));

 // method 1
 assert_eq!("2014-11-28T12:00:09Z".parse::<DateTime<Utc>>(), Ok(dt.clone()));
 assert_eq!("2014-11-28T21:00:09+09:00".parse::<DateTime<Utc>>(), Ok(dt.clone()));
 assert_eq!("2014-11-28T21:00:09+09:00".parse::<DateTime<FixedOffset>>(), Ok(fixed_dt.clone()));

 // method 2
 assert_eq!(DateTime::parse_from_str("2014-11-28 21:00:09 +09:00", "%Y-%m-%d %H:%M:%S %z"),
            Ok(fixed_dt.clone()));
 assert_eq!(DateTime::parse_from_rfc2822("Fri, 28 Nov 2014 21:00:09 +0900"),
            Ok(fixed_dt.clone()));
 assert_eq!(DateTime::parse_from_rfc3339("2014-11-28T21:00:09+09:00"), Ok(fixed_dt.clone()));

 // method 3
 assert_eq!(Utc.datetime_from_str("2014-11-28 12:00:09", "%Y-%m-%d %H:%M:%S"), Ok(dt.clone()));
 assert_eq!(Utc.datetime_from_str("Fri Nov 28 12:00:09 2014", "%a %b %e %T %Y"), Ok(dt.clone()));

 // oops, the year is missing!
 assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T %Y").is_err());
 // oops, the format string does not include the year at all!
 assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T").is_err());
 // oops, the weekday is incorrect!
 assert!(Utc.datetime_from_str("Sat Nov 28 12:00:09 2014", "%a %b %e %T %Y").is_err());
 ```

 Again : See [`format::strftime`](https://docs.rs/chrono/0.4/chrono/format/strftime/index.html#specifiers)
 documentation for full syntax and list of specifiers.

 ### Conversion from and to EPOCH timestamps

 Use [`Utc.timestamp(seconds, nanoseconds)`](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html#method.timestamp)
 to construct a [`DateTime<Utc>`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html) from a UNIX timestamp
 (seconds, nanoseconds that passed since January 1st 1970).

 Use [`DateTime.timestamp`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.timestamp) to get the timestamp (in seconds)
 from a [`DateTime`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html). Additionally, you can use
 [`DateTime.timestamp_subsec_nanos`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.timestamp_subsec_nanos)
 to get the number of additional number of nanoseconds.

 ```rust
 // We need the trait in scope to use Utc::timestamp().
 use chrono::{DateTime, TimeZone, Utc};

 // Construct a datetime from epoch:
 let dt = Utc.timestamp(1_500_000_000, 0);
 assert_eq!(dt.to_rfc2822(), "Fri, 14 Jul 2017 02:40:00 +0000");

 // Get epoch value from a datetime:
 let dt = DateTime::parse_from_rfc2822("Fri, 14 Jul 2017 02:40:00 +0000").unwrap();
 assert_eq!(dt.timestamp(), 1_500_000_000);
 ```

 ### Individual date

 Chrono also provides an individual date type ([**`Date`**](https://docs.rs/chrono/0.4/chrono/struct.Date.html)).
 It also has time zones attached, and have to be constructed via time zones.
 Most operations available to `DateTime` are also available to `Date` whenever appropriate.

 ```rust
 use chrono::prelude::*;
 use chrono::offset::LocalResult;

 assert_eq!(Utc::today(), Utc::now().date());
 assert_eq!(Local::today(), Local::now().date());

 assert_eq!(Utc.ymd(2014, 11, 28).weekday(), Weekday::Fri);
 assert_eq!(Utc.ymd_opt(2014, 11, 31), LocalResult::None);
 assert_eq!(Utc.ymd(2014, 11, 28).and_hms_milli(7, 8, 9, 10).format("%H%M%S").to_string(),
            "070809");
 ```

 There is no timezone-aware `Time` due to the lack of usefulness and also the complexity.

 `DateTime` has [`date`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.date) method
 which returns a `Date` which represents its date component.
 There is also a [`time`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.time) method,
 which simply returns a naive local time described below.

 ### Naive date and time

 Chrono provides naive counterparts to `Date`, (non-existent) `Time` and `DateTime`
 as [**`NaiveDate`**](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveDate.html),
 [**`NaiveTime`**](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveTime.html) and
 [**`NaiveDateTime`**](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveDateTime.html) respectively.

 They have almost equivalent interfaces as their timezone-aware twins,
 but are not associated to time zones obviously and can be quite low-level.
 They are mostly useful for building blocks for higher-level types.

 Timezone-aware `DateTime` and `Date` types have two methods returning naive versions:
 [`naive_local`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.naive_local) returns
 a view to the naive local time,
 and [`naive_utc`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.naive_utc) returns
 a view to the naive UTC time.

 ## Limitations

 Only proleptic Gregorian calendar (i.e. extended to support older dates) is supported.
 Be very careful if you really have to deal with pre-20C dates, they can be in Julian or others.

 Date types are limited in about +/- 262,000 years from the common epoch.
 Time types are limited in the nanosecond accuracy.

 [Leap seconds are supported in the representation but
 Chrono doesn't try to make use of them](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveTime.html#leap-second-handling).
 (The main reason is that leap seconds are not really predictable.)
 Almost *every* operation over the possible leap seconds will ignore them.
 Consider using `NaiveDateTime` with the implicit TAI (International Atomic Time) scale
 if you want.

 Chrono inherently does not support an inaccurate or partial date and time representation.
 Any operation that can be ambiguous will return `None` in such cases.
 For example, "a month later" of 2014-01-30 is not well-defined
 and consequently `Utc.ymd(2014, 1, 30).with_month(2)` returns `None`.

 Non ISO week handling is not yet supported.
 For now you can use the [chrono_ext](https://crates.io/crates/chrono_ext)
 crate ([sources](https://github.com/bcourtine/chrono-ext/)).

 Advanced time zone handling is not yet supported.
 For now you can try the [Chrono-tz](https://github.com/chronotope/chrono-tz/) crate instead.
	[Chrono][docsrs]: Date and Time for Rust
	========================================

	[![Chrono GitHub Actions][gh-image]][gh-checks]
	[![Chrono on crates.io][cratesio-image]][cratesio]
	[![Chrono on docs.rs][docsrs-image]][docsrs]
	[![Join the chat at https://gitter.im/chrono-rs/chrono][gitter-image]][gitter]

	[gh-image]: https://github.com/chronotope/chrono/workflows/test/badge.svg
	[gh-checks]: https://github.com/chronotope/chrono/actions?query=workflow%3Atest
	[cratesio-image]: https://img.shields.io/crates/v/chrono.svg
	[cratesio]: https://crates.io/crates/chrono
	[docsrs-image]: https://docs.rs/chrono/badge.svg
	[docsrs]: https://docs.rs/chrono
	[gitter-image]: https://badges.gitter.im/chrono-rs/chrono.svg
	[gitter]: https://gitter.im/chrono-rs/chrono

	It aims to be a feature-complete superset of
	the [time](https://github.com/rust-lang-deprecated/time) library.
	In particular,

	* Chrono strictly adheres to ISO 8601.
	* Chrono is timezone-aware by default, with separate timezone-naive types.
	* Chrono is space-optimal and (while not being the primary goal) reasonably efficient.

	There were several previous attempts to bring a good date and time library to Rust,
	which Chrono builds upon and should acknowledge:

	* [Initial research on
	the wiki](https://github.com/rust-lang/rust-wiki-backup/blob/master/Lib-datetime.md)
	* Dietrich Epp's [datetime-rs](https://github.com/depp/datetime-rs)
	* Luis de Bethencourt's [rust-datetime](https://github.com/luisbg/rust-datetime)

	Any significant changes to Chrono are documented in
	the [`CHANGELOG.md`](https://github.com/chronotope/chrono/blob/main/CHANGELOG.md) file.


	## Usage

	Put this in your `Cargo.toml`:

	```toml
	[dependencies]
	chrono = "0.4"
	```

	### Features

	Chrono supports various runtime environments and operating systems, and has
	several features that may be enabled or disabled.

	Default features:

	- `alloc`: Enable features that depend on allocation (primarily string formatting)
	- `std`: Enables functionality that depends on the standard library. This
	is a superset of `alloc` and adds interoperation with standard library types
	and traits.
	- `clock`: enables reading the system time (`now`), independent of whether
	`std::time::SystemTime` is present, depends on having a libc.

	Optional features:

	- `wasmbind`: Enable integration with [wasm-bindgen][] and its `js-sys` project
	- [`serde`][]: Enable serialization/deserialization via serde.
	- `unstable-locales`: Enable localization. This adds various methods with a
	`_localized` suffix. The implementation and API may change or even be
	removed in a patch release. Feedback welcome.

	[`serde`]: https://github.com/serde-rs/serde
	[wasm-bindgen]: https://github.com/rustwasm/wasm-bindgen

	See the [cargo docs][] for examples of specifying features.

	[cargo docs]: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#choosing-features

	## Overview

	### Duration

	Chrono currently uses its own [`Duration`] type to represent the magnitude
	of a time span. Since this has the same name as the newer, standard type for
	duration, the reference will refer this type as `OldDuration`.

	Note that this is an "accurate" duration represented as seconds and
	nanoseconds and does not represent "nominal" components such as days or
	months.

	When the `oldtime` feature is enabled, [`Duration`] is an alias for the
	[`time::Duration`](https://docs.rs/time/0.1.40/time/struct.Duration.html)
	type from v0.1 of the time crate. time v0.1 is deprecated, so new code
	should disable the `oldtime` feature and use the `chrono::Duration` type
	instead. The `oldtime` feature is enabled by default for backwards
	compatibility, but future versions of Chrono are likely to remove the
	feature entirely.

	Chrono does not yet natively support
	the standard [`Duration`](https://doc.rust-lang.org/std/time/struct.Duration.html) type,
	but it will be supported in the future.
	Meanwhile you can convert between two types with
	[`Duration::from_std`](https://docs.rs/time/0.1.40/time/struct.Duration.html#method.from_std)
	and
	[`Duration::to_std`](https://docs.rs/time/0.1.40/time/struct.Duration.html#method.to_std)
	methods.

	### Date and Time

	Chrono provides a
	[`DateTime`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html)
	type to represent a date and a time in a timezone.

	For more abstract moment-in-time tracking such as internal timekeeping
	that is unconcerned with timezones, consider
	[`time::SystemTime`](https://doc.rust-lang.org/std/time/struct.SystemTime.html),
	which tracks your system clock, or
	[`time::Instant`](https://doc.rust-lang.org/std/time/struct.Instant.html), which
	is an opaque but monotonically-increasing representation of a moment in time.

	`DateTime` is timezone-aware and must be constructed from
	the [`TimeZone`](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html) object,
	which defines how the local date is converted to and back from the UTC date.
	There are three well-known `TimeZone` implementations:

	* [`Utc`](https://docs.rs/chrono/0.4/chrono/offset/struct.Utc.html) specifies the UTC time zone. It is most efficient.

	* [`Local`](https://docs.rs/chrono/0.4/chrono/offset/struct.Local.html) specifies the system local time zone.

	* [`FixedOffset`](https://docs.rs/chrono/0.4/chrono/offset/struct.FixedOffset.html) specifies
	an arbitrary, fixed time zone such as UTC+09:00 or UTC-10:30.
	This often results from the parsed textual date and time.
	Since it stores the most information and does not depend on the system environment,
	you would want to normalize other `TimeZone`s into this type.

	`DateTime`s with different `TimeZone` types are distinct and do not mix,
	but can be converted to each other using
	the [`DateTime::with_timezone`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.with_timezone) method.

	You can get the current date and time in the UTC time zone
	([`Utc::now()`](https://docs.rs/chrono/0.4/chrono/offset/struct.Utc.html#method.now))
	or in the local time zone
	([`Local::now()`](https://docs.rs/chrono/0.4/chrono/offset/struct.Local.html#method.now)).

	```rust
	use chrono::prelude::*;

	let utc: DateTime<Utc> = Utc::now(); // e.g. `2014-11-28T12:45:59.324310806Z`
	let local: DateTime<Local> = Local::now(); // e.g. `2014-11-28T21:45:59.324310806+09:00`
	```

	Alternatively, you can create your own date and time.
	This is a bit verbose due to Rust's lack of function and method overloading,
	but in turn we get a rich combination of initialization methods.

	```rust
	use chrono::prelude::*;
	use chrono::offset::LocalResult;

	let dt = Utc.ymd(2014, 7, 8).and_hms(9, 10, 11); // `2014-07-08T09:10:11Z`
	// July 8 is 188th day of the year 2014 (`o` for "ordinal")
	assert_eq!(dt, Utc.yo(2014, 189).and_hms(9, 10, 11));
	// July 8 is Tuesday in ISO week 28 of the year 2014.
	assert_eq!(dt, Utc.isoywd(2014, 28, Weekday::Tue).and_hms(9, 10, 11));

	let dt = Utc.ymd(2014, 7, 8).and_hms_milli(9, 10, 11, 12); // `2014-07-08T09:10:11.012Z`
	assert_eq!(dt, Utc.ymd(2014, 7, 8).and_hms_micro(9, 10, 11, 12_000));
	assert_eq!(dt, Utc.ymd(2014, 7, 8).and_hms_nano(9, 10, 11, 12_000_000));

	// dynamic verification
	assert_eq!(Utc.ymd_opt(2014, 7, 8).and_hms_opt(21, 15, 33),
	LocalResult::Single(Utc.ymd(2014, 7, 8).and_hms(21, 15, 33)));
	assert_eq!(Utc.ymd_opt(2014, 7, 8).and_hms_opt(80, 15, 33), LocalResult::None);
	assert_eq!(Utc.ymd_opt(2014, 7, 38).and_hms_opt(21, 15, 33), LocalResult::None);

	// other time zone objects can be used to construct a local datetime.
	// obviously, `local_dt` is normally different from `dt`, but `fixed_dt` should be identical.
	let local_dt = Local.ymd(2014, 7, 8).and_hms_milli(9, 10, 11, 12);
	let fixed_dt = FixedOffset::east(9 * 3600).ymd(2014, 7, 8).and_hms_milli(18, 10, 11, 12);
	assert_eq!(dt, fixed_dt);
	```

	Various properties are available to the date and time, and can be altered individually.
	Most of them are defined in the traits [`Datelike`](https://docs.rs/chrono/0.4/chrono/trait.Datelike.html) and
	[`Timelike`](https://docs.rs/chrono/0.4/chrono/trait.Timelike.html) which you should `use` before.
	Addition and subtraction is also supported.
	The following illustrates most supported operations to the date and time:

	```rust

	use chrono::prelude::*;
	use chrono::Duration;

	// assume this returned `2014-11-28T21:45:59.324310806+09:00`:
	let dt = FixedOffset::east(9*3600).ymd(2014, 11, 28).and_hms_nano(21, 45, 59, 324310806);

	// property accessors
	assert_eq!((dt.year(), dt.month(), dt.day()), (2014, 11, 28));
	assert_eq!((dt.month0(), dt.day0()), (10, 27)); // for unfortunate souls
	assert_eq!((dt.hour(), dt.minute(), dt.second()), (21, 45, 59));
	assert_eq!(dt.weekday(), Weekday::Fri);
	assert_eq!(dt.weekday().number_from_monday(), 5); // Mon=1, ..., Sun=7
	assert_eq!(dt.ordinal(), 332); // the day of year
	assert_eq!(dt.num_days_from_ce(), 735565); // the number of days from and including Jan 1, 1

	// time zone accessor and manipulation
	assert_eq!(dt.offset().fix().local_minus_utc(), 9 * 3600);
	assert_eq!(dt.timezone(), FixedOffset::east(9 * 3600));
	assert_eq!(dt.with_timezone(&Utc), Utc.ymd(2014, 11, 28).and_hms_nano(12, 45, 59, 324310806));

	// a sample of property manipulations (validates dynamically)
	assert_eq!(dt.with_day(29).unwrap().weekday(), Weekday::Sat); // 2014-11-29 is Saturday
	assert_eq!(dt.with_day(32), None);
	assert_eq!(dt.with_year(-300).unwrap().num_days_from_ce(), -109606); // November 29, 301 BCE

	// arithmetic operations
	let dt1 = Utc.ymd(2014, 11, 14).and_hms(8, 9, 10);
	let dt2 = Utc.ymd(2014, 11, 14).and_hms(10, 9, 8);
	assert_eq!(dt1.signed_duration_since(dt2), Duration::seconds(-2 * 3600 + 2));
	assert_eq!(dt2.signed_duration_since(dt1), Duration::seconds(2 * 3600 - 2));
	assert_eq!(Utc.ymd(1970, 1, 1).and_hms(0, 0, 0) + Duration::seconds(1_000_000_000),
	Utc.ymd(2001, 9, 9).and_hms(1, 46, 40));
	assert_eq!(Utc.ymd(1970, 1, 1).and_hms(0, 0, 0) - Duration::seconds(1_000_000_000),
	Utc.ymd(1938, 4, 24).and_hms(22, 13, 20));
	```

	### Formatting and Parsing

	Formatting is done via the [`format`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.format) method,
	which format is equivalent to the familiar `strftime` format.

	See [`format::strftime`](https://docs.rs/chrono/0.4/chrono/format/strftime/index.html#specifiers)
	documentation for full syntax and list of specifiers.

	The default `to_string` method and `{:?}` specifier also give a reasonable representation.
	Chrono also provides [`to_rfc2822`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.to_rfc2822) and
	[`to_rfc3339`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.to_rfc3339) methods
	for well-known formats.

	Chrono now also provides date formatting in almost any language without the
	help of an additional C library. This functionality is under the feature
	`unstable-locales`:

	```text
	chrono { version = "0.4", features = ["unstable-locales"]
	```

	The `unstable-locales` feature requires and implies at least the `alloc` feature.

	```rust
	use chrono::prelude::*;

	let dt = Utc.ymd(2014, 11, 28).and_hms(12, 0, 9);
	assert_eq!(dt.format("%Y-%m-%d %H:%M:%S").to_string(), "2014-11-28 12:00:09");
	assert_eq!(dt.format("%a %b %e %T %Y").to_string(), "Fri Nov 28 12:00:09 2014");
	assert_eq!(dt.format_localized("%A %e %B %Y, %T", Locale::fr_BE).to_string(), "vendredi 28 novembre 2014, 12:00:09");
	assert_eq!(dt.format("%a %b %e %T %Y").to_string(), dt.format("%c").to_string());

	assert_eq!(dt.to_string(), "2014-11-28 12:00:09 UTC");
	assert_eq!(dt.to_rfc2822(), "Fri, 28 Nov 2014 12:00:09 +0000");
	assert_eq!(dt.to_rfc3339(), "2014-11-28T12:00:09+00:00");
	assert_eq!(format!("{:?}", dt), "2014-11-28T12:00:09Z");

	// Note that milli/nanoseconds are only printed if they are non-zero
	let dt_nano = Utc.ymd(2014, 11, 28).and_hms_nano(12, 0, 9, 1);
	assert_eq!(format!("{:?}", dt_nano), "2014-11-28T12:00:09.000000001Z");
	```

	Parsing can be done with three methods:

	1. The standard [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) trait
	(and [`parse`](https://doc.rust-lang.org/std/primitive.str.html#method.parse) method
	on a string) can be used for parsing `DateTime<FixedOffset>`, `DateTime<Utc>` and
	`DateTime<Local>` values. This parses what the `{:?}`
	([`std::fmt::Debug`](https://doc.rust-lang.org/std/fmt/trait.Debug.html))
	format specifier prints, and requires the offset to be present.

	2. [`DateTime::parse_from_str`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_str) parses
	a date and time with offsets and returns `DateTime<FixedOffset>`.
	This should be used when the offset is a part of input and the caller cannot guess that.
	It cannot be used when the offset can be missing.
	[`DateTime::parse_from_rfc2822`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_rfc2822)
	and
	[`DateTime::parse_from_rfc3339`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.parse_from_rfc3339)
	are similar but for well-known formats.

	3. [`Offset::datetime_from_str`](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html#method.datetime_from_str) is
	similar but returns `DateTime` of given offset.
	When the explicit offset is missing from the input, it simply uses given offset.
	It issues an error when the input contains an explicit offset different
	from the current offset.

	More detailed control over the parsing process is available via
	[`format`](https://docs.rs/chrono/0.4/chrono/format/index.html) module.

	```rust
	use chrono::prelude::*;

	let dt = Utc.ymd(2014, 11, 28).and_hms(12, 0, 9);
	let fixed_dt = dt.with_timezone(&FixedOffset::east(9*3600));

	// method 1
	assert_eq!("2014-11-28T12:00:09Z".parse::<DateTime<Utc>>(), Ok(dt.clone()));
	assert_eq!("2014-11-28T21:00:09+09:00".parse::<DateTime<Utc>>(), Ok(dt.clone()));
	assert_eq!("2014-11-28T21:00:09+09:00".parse::<DateTime<FixedOffset>>(), Ok(fixed_dt.clone()));

	// method 2
	assert_eq!(DateTime::parse_from_str("2014-11-28 21:00:09 +09:00", "%Y-%m-%d %H:%M:%S %z"),
	Ok(fixed_dt.clone()));
	assert_eq!(DateTime::parse_from_rfc2822("Fri, 28 Nov 2014 21:00:09 +0900"),
	Ok(fixed_dt.clone()));
	assert_eq!(DateTime::parse_from_rfc3339("2014-11-28T21:00:09+09:00"), Ok(fixed_dt.clone()));

	// method 3
	assert_eq!(Utc.datetime_from_str("2014-11-28 12:00:09", "%Y-%m-%d %H:%M:%S"), Ok(dt.clone()));
	assert_eq!(Utc.datetime_from_str("Fri Nov 28 12:00:09 2014", "%a %b %e %T %Y"), Ok(dt.clone()));

	// oops, the year is missing!
	assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T %Y").is_err());
	// oops, the format string does not include the year at all!
	assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T").is_err());
	// oops, the weekday is incorrect!
	assert!(Utc.datetime_from_str("Sat Nov 28 12:00:09 2014", "%a %b %e %T %Y").is_err());
	```

	Again : See [`format::strftime`](https://docs.rs/chrono/0.4/chrono/format/strftime/index.html#specifiers)
	documentation for full syntax and list of specifiers.

	### Conversion from and to EPOCH timestamps

	Use [`Utc.timestamp(seconds, nanoseconds)`](https://docs.rs/chrono/0.4/chrono/offset/trait.TimeZone.html#method.timestamp)
	to construct a [`DateTime<Utc>`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html) from a UNIX timestamp
	(seconds, nanoseconds that passed since January 1st 1970).

	Use [`DateTime.timestamp`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.timestamp) to get the timestamp (in seconds)
	from a [`DateTime`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html). Additionally, you can use
	[`DateTime.timestamp_subsec_nanos`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.timestamp_subsec_nanos)
	to get the number of additional number of nanoseconds.

	```rust
	// We need the trait in scope to use Utc::timestamp().
	use chrono::{DateTime, TimeZone, Utc};

	// Construct a datetime from epoch:
	let dt = Utc.timestamp(1_500_000_000, 0);
	assert_eq!(dt.to_rfc2822(), "Fri, 14 Jul 2017 02:40:00 +0000");

	// Get epoch value from a datetime:
	let dt = DateTime::parse_from_rfc2822("Fri, 14 Jul 2017 02:40:00 +0000").unwrap();
	assert_eq!(dt.timestamp(), 1_500_000_000);
	```

	### Individual date

	Chrono also provides an individual date type ([`Date`](https://docs.rs/chrono/0.4/chrono/struct.Date.html)).
	It also has time zones attached, and have to be constructed via time zones.
	Most operations available to `DateTime` are also available to `Date` whenever appropriate.

	```rust
	use chrono::prelude::*;
	use chrono::offset::LocalResult;

	assert_eq!(Utc::today(), Utc::now().date());
	assert_eq!(Local::today(), Local::now().date());

	assert_eq!(Utc.ymd(2014, 11, 28).weekday(), Weekday::Fri);
	assert_eq!(Utc.ymd_opt(2014, 11, 31), LocalResult::None);
	assert_eq!(Utc.ymd(2014, 11, 28).and_hms_milli(7, 8, 9, 10).format("%H%M%S").to_string(),
	"070809");
	```

	There is no timezone-aware `Time` due to the lack of usefulness and also the complexity.

	`DateTime` has [`date`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.date) method
	which returns a `Date` which represents its date component.
	There is also a [`time`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.time) method,
	which simply returns a naive local time described below.

	### Naive date and time

	Chrono provides naive counterparts to `Date`, (non-existent) `Time` and `DateTime`
	as [`NaiveDate`](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveDate.html),
	[`NaiveTime`](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveTime.html) and
	[`NaiveDateTime`](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveDateTime.html) respectively.

	They have almost equivalent interfaces as their timezone-aware twins,
	but are not associated to time zones obviously and can be quite low-level.
	They are mostly useful for building blocks for higher-level types.

	Timezone-aware `DateTime` and `Date` types have two methods returning naive versions:
	[`naive_local`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.naive_local) returns
	a view to the naive local time,
	and [`naive_utc`](https://docs.rs/chrono/0.4/chrono/struct.DateTime.html#method.naive_utc) returns
	a view to the naive UTC time.

	## Limitations

	Only proleptic Gregorian calendar (i.e. extended to support older dates) is supported.
	Be very careful if you really have to deal with pre-20C dates, they can be in Julian or others.

	Date types are limited in about +/- 262,000 years from the common epoch.
	Time types are limited in the nanosecond accuracy.

	[Leap seconds are supported in the representation but
	Chrono doesn't try to make use of them](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveTime.html#leap-second-handling).
	(The main reason is that leap seconds are not really predictable.)
	Almost every operation over the possible leap seconds will ignore them.
	Consider using `NaiveDateTime` with the implicit TAI (International Atomic Time) scale
	if you want.

	Chrono inherently does not support an inaccurate or partial date and time representation.
	Any operation that can be ambiguous will return `None` in such cases.
	For example, "a month later" of 2014-01-30 is not well-defined
	and consequently `Utc.ymd(2014, 1, 30).with_month(2)` returns `None`.

	Non ISO week handling is not yet supported.
	For now you can use the [chrono_ext](https://crates.io/crates/chrono_ext)
	crate ([sources](https://github.com/bcourtine/chrono-ext/)).

	Advanced time zone handling is not yet supported.
	For now you can try the [Chrono-tz](https://github.com/chronotope/chrono-tz/) crate instead.