chrono breaking changes

• 0.3.1 \u2192 0.4.0breaking

  The `+=` and `-=` operators against `time::Duration` are now supported for

• 0.3.1 \u2192 0.4.0breaking

  `IsoWeek` has been added for the ISO week without time zone.

• 0.3.1 \u2192 0.4.0breaking

  `SystemTime` and `DateTime<Tz>` types can be now converted to each other via `From`.

• 0.3.1 \u2192 0.4.0breaking

  Intermediate implementation modules have been flattened (#161),

• 0.3.1 \u2192 0.4.0breaking

  `Datelike::isoweekdate` is replaced by `Datelike::iso_week` which only returns the ISO week.

• 0.3.1 \u2192 0.4.0breaking

  `NaiveDateTime` and `DateTime` can now be deserialized from an integral UNIX timestamp. (#125)

• 0.3.1 \u2192 0.4.0breaking

  `chrono::serde` and `chrono::naive::serde` modules have been added

• 0.3.1 \u2192 0.4.0breaking

  The major version was made to fix the broken Serde dependency issues. (#146, #156, #158, #159)

• 0.3.1 \u2192 0.4.0breaking

  Various documentation fixes and goodies. (#92, #131, #136)

• 0.3.0 \u2192 0.3.1breaking

  Serde 1.0 is now supported. (#142)

• 0.3.0 \u2192 0.3.1breaking

  `Weekday` now implements `FromStr`, `Serialize` and `Deserialize`. (#113)

• 0.3.0 \u2192 0.3.1breaking

  Fixed a bug that the leap second can be mapped wrongly in the local time zone.

• 0.2.25 \u2192 0.3.0breaking

  Rustc-serialize support for `Date<Tz>` types and all offset types has been dropped.

• 0.2.25 \u2192 0.3.0breaking

  `NaiveDateTime::num_seconds_unix_epoch` (→ `NaiveDateTime::timestamp`)

• 0.2.25 \u2192 0.3.0breaking

  Formatting items are no longer `Copy`, except for `chrono::format::Pad`.

• 0.2.25 \u2192 0.3.0breaking

  `DateTime::num_seconds_from_unix_epoch` (→ `DateTime::timestamp`)

• 0.2.25 \u2192 0.3.0breaking

  `NaiveDateTime::from_num_seconds_from_unix_epoch` (→ `NaiveDateTime::from_timestamp`)

• 0.2.25 \u2192 0.3.0breaking

  `NaiveDateTime::from_num_seconds_from_unix_epoch_opt` (→ `NaiveDateTime::from_timestamp_opt`)

• 0.2.25 \u2192 0.3.0removed

  The following method aliases deprecated in the 0.2 branch have been removed.

• 0.2.25 \u2192 0.3.0removed

  `chrono::offset::add_with_leapsecond` has been removed.

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