Documentation update for DateTime
Documentation update for DateTime

Examples updated and new ones added.

Closes https://github.com/dart-lang/sdk/pull/47812
https://github.com/dart-lang/sdk/pull/47812

GitOrigin-RevId: dc23ccf574845e4fa8b9126bdf3163c487ce48f5
Change-Id: I08277cd671c6381959fabce359500fd6dfa3f17f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221603
Reviewed-by: Lasse Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

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