How to get start of or end of week in dart

Question

How can I get the start of or end of a week in dart? An example is if three days ago was a Monday, and today is a Wednesday, how can I find the start of the week using dart, that is on Monday

Answer 1

Dart DateTimes have a weekday getter which is 1 for Monday and 7 for Sunday. Using that, I would do:

DateTime mostRecentSunday(DateTime date) =>
    DateTime(date.year, date.month, date.day - date.weekday % 7);

to get the most recent Sunday (which is the start of the current week if the week starts on a Sunday), and

DateTime mostRecentMonday(DateTime date) =>
    DateTime(date.year, date.month, date.day - (date.weekday - 1));

for the most recent Monday (which is then the start of the current week if the week starts on a Monday).

You can generalize to

/// The [weekday] may be 0 for Sunday, 1 for Monday, etc. up to 7 for Sunday.
DateTime mostRecentWeekday(DateTime date, int weekday) =>
    DateTime(date.year, date.month, date.day - (date.weekday - weekday) % 7);

If you are going to be using the results as calendar dates, I'd used DateTime.utc as the constructor instead. (Always use UTC for calendar dates, then you can do day-based arithmetic on them safely).

I'd even consider using DateTime.utc in any case because it avoids any potential issues with a daylight saving that starts at midnight (rare, but with time zones, even unlikely things tend to have happened somewhere at some point).

Answer 2

You can get the weekday from the DateTime using https://api.dart.dev/stable/2.5.1/dart-core/DateTime/weekday.html and add/subtract this number from you date:

void main() {
  final date = DateTime.parse('2019-10-08 15:43:03.887');
  
  print('Date: $date');
  print('Start of week: ${getDate(date.subtract(Duration(days: date.weekday - 1)))}');
  print('End of week: ${getDate(date.add(Duration(days: DateTime.daysPerWeek - date.weekday)))}');
}

DateTime getDate(DateTime d) => DateTime(d.year, d.month, d.day);

UPDATE

Please read and upvote the answer from lrn. He knows a lot more about this stuff than me. :)