New methods on DateTime (lastDayOfMonth, lastDayOfWeek, firstDayOfWeek)

[FMorschel]

Looking for lastDayOfMonth like feature, found this StackOverflow question and I really liked the way Yogesh Parwani mixed Chris Buckett and Kai Sellgren awnsers. As pointed by Juniper Belmont on Buckett's answer, I do think this could be a new feature or at least pointed on the official documentation that these values could be obtained like this.

Code bellow:

extension DateTimeExtension on DateTime {

  DateTime get firstDayOfWeek => subtract(Duration(days: weekday - 1));

  DateTime get lastDayOfWeek => add(Duration(days: DateTime.daysPerWeek - weekday));

  DateTime get lastDayOfMonth => month < 12 ? DateTime(year, month + 1, 0) : DateTime(year + 1, 1, 0);
}

[natebosch]

cc @lrhn for thoughts

My initial reaction is we probably wouldn't put this in the SDK. I could imagine putting this into a utility package, but it's not a good fit for any of our existing core packages.

[lrhn]

We don't have a package:core, which is where it would fit.

I'm not completely opposed to putting DateTime extensions in dart:core, but I don't see this as features that will be widely used, which is one of the criteria for going into a dart: library (widely used by many users, or heavily used by some users, and something you'll only ever need one canonical version of, so we might as well provide it - or something which requires native interop with the runtime system).

The "firstDayOfWeek" is contentious, since some people (incorrectly 😛) consider Sunday the first day of the week, even though it is obviously Monday. That suggests having separate packages that you can choose between.

Maybe we should have a package:calendar (name currently taken by a 4-year-stale package.)

(Also, don't use Duration when calculating dates. Better implementation:

extension DateTimeFirstLast on DateTime {
  DateTime get firstDayOfWeek => DateTime.utc(year, month, day + 1 - weekday);
  DateTime get lastDayOfWeek => DateTime.utc(year, month, day + 7 - weekday);  
  DateTime get firstDayOfMonth => DateTime.utc(year, month, 1);
  DateTime get lastDayOfMonth => DateTime.utc(year, month + 1, 0);
  DateTime get firstDayOfYear => DateTime.utc(year, 1, 1);
  DateTime get lastDayOfYear => DateTime.utc(year, 12, 31);
}

which isn't affected by the time of the DateTime object it's called on, and uses UTC to represent time-less dates as recommended.)

[mnordine]

@FMorschel you should submit a PR to the time package: https://github.com/jogboms/time.dart

[FMorschel]

As this request is now on https://github.com/jogboms/time.dart as suggested, I'm closing this issue.