NSCalendar, why does setting the firstWeekday doesn't effect calculation outcome?

Question

i need to calculate the weekday for a given date, however, depending on the calendar a week can star on Monday and somwhere on Sunday

so i wanted to set it, to start on Monday, using

[[NSCalendar currentCalendar] setFirstWeekday:2];

however, the calculation outcome is the same

{
    [[NSCalendar currentCalendar] setFirstWeekday:1];
    NSDateComponents *weekdayComponents = [[NSCalendar currentCalendar] components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];
    NSInteger weekday = [weekdayComponents weekday] - 1;
    NSLog(@"%d", weekday);
}
{
    [[NSCalendar currentCalendar] setFirstWeekday:2];
    NSDateComponents *weekdayComponents = [[NSCalendar currentCalendar] components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];
    NSInteger weekday = [weekdayComponents weekday] - 1;
    NSLog(@"%d", weekday);
}

returns same numbers, but why?

score 11 · Accepted Answer · answered Jun 01 '13 at 19:27

11

The behavior you see is correct. The weekday component is not affected by the firstWeekday property. If you have a date representing a sunday it will always be a sunday wether you start your week on that sunday or on monday. What this should affect is the week number in the week property of your date components.

answered Jun 01 '13 at 19:27

Sven

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so Sunday will always be weekday 0, no matter what the firstWeekday points? – Peter Lapisu Jun 01 '13 at 19:44
3

Sunday 1, Monday 2, Tuesday 3, Wednesday 4, Friday 5, Saturday 6 – Peter Lapisu Jun 01 '13 at 20:20
16

@Peter Lapisu dont forget Thur! :-) – marchinram Nov 04 '13 at 05:28
2

In details, NSCalendar's `firstWeekDay` just influences `NSDateComponents`'s `weekOfMonth/weekOfYear/yearForWeekOfYear` property. By the way, NSCalendar's `week` property is deprecated, and substituted by `weekOfMonth` and `weekOfYear`. – DawnSong May 03 '16 at 09:40

score 5 · Answer 2 · edited Dec 11 '16 at 07:43

5

I believe you need to use the ordinalityOfUnit:inUnit:forDate: method rather than attempting to extract the date components. So something like this:

NSUInteger weekday = [[NSCalendar currentCalendar] ordinalityOfUnit:NSWeekdayCalendarUnit inUnit:NSWeekCalendarUnit forDate:date];

Basically that call is asking for the day (NSWeekdayCalendarUnit) in the week (NSWeekCalendarUnit) for the given date.

If that doesn't work as is, you may need to create your own calendar, rather than trying to modifying the first week day on the currentCalendar.

For example:

NSCalendarIdentifier calendarIdentifier = [[NSCalendar currentCalendar] calendarIdentifier];
NSCalendar *calendar = [[[NSCalendar alloc] initWithCalendarIdentifier:calendarIdentifier] autorelease];
[calendar setFirstWeekday:2];

Then use that new calendar object rather than [NSCalendar currentCalendar] in the ordinalityOfUnit:inUnit:forDate: call.

edited Dec 11 '16 at 07:43

Dmitry

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answered Jun 01 '13 at 19:20

James Holderness

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NSWeekCalendarUnit is deprecated :/ – Peter Lapisu Jun 01 '13 at 19:46
Perhaps try one of the other week units: `NSWeekOfMonthCalendarUnit` or `NSWeekOfYearCalendarUnit`. They don't really make sense in this context, but it's worth trying. – James Holderness Jun 01 '13 at 20:04
If you're finding you're getting strange values when simply plugging in `NSWeekOfMonthCalendarUnit` or `NSWeekOfYearCalendarUnit`, try... `NSDateComponents *components = [[NSCalendar currentCalendar] components:NSCalendarUnitWeekday fromDate:[NSDate date]];` – roycable Apr 04 '18 at 04:01

score 0 · Answer 3 · answered Jun 30 '18 at 10:18

A useful function to get corrected weekday number for different firstWeekday cases

Swift 3.0

func dayOfWeek(day: Int, month: Int, year: Int) -> Int {
    let calendar = Calendar.current
    let dateComponents = DateComponents(year: year, month: month, day: day)

    guard let date = calendar.date(from: dateComponents) else {
        fatalError("Can't create date form specified date components")
    }

    var weekday = calendar.component(.weekday, from: date)

    //handling the case when calendar starts from Monday: firstWeekday == 2
    if calendar.firstWeekday == 2 {
        weekday = (weekday == 1) ? 7 : (weekday - 1)
    }

    return weekday
}

NSCalendar, why does setting the firstWeekday doesn't effect calculation outcome?

3 Answers3

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