NSCalendar: Problem getting weeks in a month

Question

I am creating a calendar control of sorts...

One thing I need to know is how many weeks are there in a Month...

So NSCalendar rangeOfUnit:inUnit:forDate Seems to be exactly what I need...

Except I am noticing something that seems off and I can't quite figure out why this is happening...

The following code...

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
[dateComponents setYear: 2010];
[dateComponents setDay: 1];

for (int x=1; x<=12; x++)
{
    [dateComponents setMonth: x];

    NSDate *date = [calendar dateFromComponents:dateComponents];
    NSLog(@"Date: %@", date);

    NSRange range = [calendar rangeOfUnit: NSWeekCalendarUnit
                                   inUnit: NSMonthCalendarUnit
                                  forDate:date];

    NSLog(@"%d Weeks in Month %d", range.length, [dateComponents month]);

}

Is returning the following debug messages...

2010-03-14 13:08:10.350 Scrap[4256:207] Date: 2010-01-01 00:00:00 -0500
2010-03-14 13:08:10.351 Scrap[4256:207] 5 Weeks in Month 1
2010-03-14 13:08:10.352 Scrap[4256:207] Date: 2010-02-01 00:00:00 -0500
2010-03-14 13:08:10.352 Scrap[4256:207] 4 Weeks in Month 2
2010-03-14 13:08:10.353 Scrap[4256:207] Date: 2010-03-01 00:00:00 -0500
2010-03-14 13:08:10.353 Scrap[4256:207] 5 Weeks in Month 3
2010-03-14 13:08:10.354 Scrap[4256:207] Date: 2010-04-01 00:00:00 -0400
2010-03-14 13:08:10.355 Scrap[4256:207] 5 Weeks in Month 4
2010-03-14 13:08:10.356 Scrap[4256:207] Date: 2010-05-01 00:00:00 -0400
2010-03-14 13:08:10.357 Scrap[4256:207] 5 Weeks in Month 5
2010-03-14 13:08:10.358 Scrap[4256:207] Date: 2010-06-01 00:00:00 -0400
2010-03-14 13:08:10.358 Scrap[4256:207] 5 Weeks in Month 6
2010-03-14 13:08:10.359 Scrap[4256:207] Date: 2010-07-01 00:00:00 -0400
2010-03-14 13:08:10.360 Scrap[4256:207] 5 Weeks in Month 7
2010-03-14 13:08:10.361 Scrap[4256:207] Date: 2010-08-01 00:00:00 -0400
2010-03-14 13:08:10.364 Scrap[4256:207] 5 Weeks in Month 8
2010-03-14 13:08:10.364 Scrap[4256:207] Date: 2010-09-01 00:00:00 -0400
2010-03-14 13:08:10.365 Scrap[4256:207] 5 Weeks in Month 9
2010-03-14 13:08:10.366 Scrap[4256:207] Date: 2010-10-01 00:00:00 -0400
2010-03-14 13:08:10.366 Scrap[4256:207] 5 Weeks in Month 10
2010-03-14 13:08:10.367 Scrap[4256:207] Date: 2010-11-01 00:00:00 -0400
2010-03-14 13:08:10.367 Scrap[4256:207] 5 Weeks in Month 11
2010-03-14 13:08:10.369 Scrap[4256:207] Date: 2010-12-01 00:00:00 -0500
2010-03-14 13:08:10.369 Scrap[4256:207] 52 Weeks in Month 12

I cant quite figure out why I get 52 weeks in month 12.

Any clues?

Edit on 3/20/2010:

Seeing as how I couldnt use rangeOfUnit:inUnit:forDate to calculate the number of weeks in a month. I decided to figure out a different way of calculating the same value.

I figured I should do this in a non-Gregorian localized way, so I attempted to start with getting the number of days in a week, but I got the result of 28 days in a week. So I started writing more code to figure out why...

I wanted to make sure that the type of NSCalendar that I was playing with was in fact what I was supposed to be getting... And that led me to finding some differences...

NSCalendar *currentCalendar = [NSCalendar currentCalendar];

NSLog(@"Calendar with 'currentCalendar' Identifier: %@", 
      [currentCalendar calendarIdentifier]);

NSCalendar *calendarWithIdentifier = [[[NSCalendar alloc] initWithCalendarIdentifier: NSGregorianCalendar] autorelease];

NSLog(@"Calendar created with identifier Identifier: %@", 
      [calendarWithIdentifier calendarIdentifier]);

NSDate *now = [[NSDate alloc] init];

NSDateComponents *currentMonth = [currentCalendar components: NSMonthCalendarUnit | NSYearCalendarUnit
                                                    fromDate: now];

NSDate *currentMonthDate = [currentCalendar dateFromComponents: currentMonth];

NSRange daysInWeekRange = [currentCalendar rangeOfUnit: NSDayCalendarUnit
                                                inUnit: NSWeekCalendarUnit
                                               forDate: currentMonthDate];

NSLog(@"CurrentCalendar: Length:%u Location:%u", daysInWeekRange.length, daysInWeekRange.location);

currentMonth = [calendarWithIdentifier components: NSMonthCalendarUnit | NSYearCalendarUnit 
                                         fromDate: now];

currentMonthDate = [calendarWithIdentifier dateFromComponents: currentMonth];

daysInWeekRange = [calendarWithIdentifier rangeOfUnit: NSDayCalendarUnit
                                               inUnit: NSWeekCalendarUnit
                                              forDate: currentMonthDate];

NSLog(@"GregorianCalendar: Length:%u Location:%u", daysInWeekRange.length, daysInWeekRange.location);

And that got me the following log results...

2010-03-20 21:02:27.245 Scrap[52189:207] Calendar with 'currentCalendar' Identifier: gregorian
2010-03-20 21:02:27.246 Scrap[52189:207] Calendar created with identifier Identifier: gregorian
2010-03-20 21:02:27.248 Scrap[52189:207] CurrentCalendar: Length:28 Location:1
2010-03-20 21:02:27.249 Scrap[52189:207] GregorianCalendar: Length:7 Location:1

Taking direction from @CarlNorum's experience, I compiled the code snippet as a 10.6 Cocoa application, and I got the following...

2010-03-20 21:05:35.636 ScrapCocoa[52238:a0f] Calendar with 'currentCalendar' Identifier: gregorian
2010-03-20 21:05:35.636 ScrapCocoa[52238:a0f] Calendar created with identifier Identifier: gregorian
2010-03-20 21:05:35.637 ScrapCocoa[52238:a0f] CurrentCalendar: Length:6 Location:1
2010-03-20 21:05:35.638 ScrapCocoa[52238:a0f] GregorianCalendar: Length:7 Location:1

I saw hope in that creating a NSCalendar Instance explicitly as a Gregorian Calendar would lead to better results in my original problem... So I modified that original code

NSCalendar *currentCalendar = [NSCalendar currentCalendar];

NSLog(@"Calendar with 'currentCalendar' Identifier: %@", 
      [currentCalendar calendarIdentifier]);

NSCalendar *calendarWithIdentifier = [[[NSCalendar alloc] initWithCalendarIdentifier: NSGregorianCalendar] autorelease];

NSLog(@"Calendar created with identifier Identifier: %@", 
      [calendarWithIdentifier calendarIdentifier]);

NSDateComponents *dateComponents = [[[NSDateComponents alloc] init] autorelease];
[dateComponents setYear: 2010];
[dateComponents setDay: 1];

for (int x=1; x<=12; x++)
{
    [dateComponents setMonth: x];

    NSDate *date = [currentCalendar dateFromComponents: dateComponents];
    NSRange range = [currentCalendar rangeOfUnit: NSWeekCalendarUnit
                                          inUnit: NSMonthCalendarUnit
                                         forDate: date];

    NSLog(@"CurrentCalendar Date: %@", date);
    NSLog(@"CurrentCalendar: %d Weeks in Month %d", range.length, [dateComponents month]);

    date = [calendarWithIdentifier dateFromComponents: dateComponents];

    range = [calendarWithIdentifier rangeOfUnit: NSWeekCalendarUnit
                                         inUnit: NSMonthCalendarUnit
                                        forDate: date];

    NSLog(@"GregorianCalendar Date: %@", date);
    NSLog(@"GregorianCalendar: %d Weeks in Month %d", range.length, [dateComponents month]);

}

Unfortunately using a calendar created that way did not return a different result.

2010-03-20 21:15:40.465 Scrap[52367:207] Calendar with 'currentCalendar' Identifier: gregorian
2010-03-20 21:15:40.466 Scrap[52367:207] Calendar created with identifier Identifier: gregorian
2010-03-20 21:15:40.468 Scrap[52367:207] CurrentCalendar Date: 2010-01-01 00:00:00 -0500
2010-03-20 21:15:40.468 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 1
2010-03-20 21:15:40.469 Scrap[52367:207] GregorianCalendar Date: 2010-01-01 00:00:00 -0500
2010-03-20 21:15:40.470 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 1
2010-03-20 21:15:40.471 Scrap[52367:207] CurrentCalendar Date: 2010-02-01 00:00:00 -0500
2010-03-20 21:15:40.471 Scrap[52367:207] CurrentCalendar: 4 Weeks in Month 2
2010-03-20 21:15:40.472 Scrap[52367:207] GregorianCalendar Date: 2010-02-01 00:00:00 -0500
2010-03-20 21:15:40.473 Scrap[52367:207] GregorianCalendar: 4 Weeks in Month 2
2010-03-20 21:15:40.473 Scrap[52367:207] CurrentCalendar Date: 2010-03-01 00:00:00 -0500
2010-03-20 21:15:40.474 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 3
2010-03-20 21:15:40.475 Scrap[52367:207] GregorianCalendar Date: 2010-03-01 00:00:00 -0500
2010-03-20 21:15:40.475 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 3
2010-03-20 21:15:40.476 Scrap[52367:207] CurrentCalendar Date: 2010-04-01 00:00:00 -0400
2010-03-20 21:15:40.477 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 4
2010-03-20 21:15:40.478 Scrap[52367:207] GregorianCalendar Date: 2010-04-01 00:00:00 -0400
2010-03-20 21:15:40.479 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 4
2010-03-20 21:15:40.480 Scrap[52367:207] CurrentCalendar Date: 2010-05-01 00:00:00 -0400
2010-03-20 21:15:40.480 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 5
2010-03-20 21:15:40.482 Scrap[52367:207] GregorianCalendar Date: 2010-05-01 00:00:00 -0400
2010-03-20 21:15:40.482 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 5
2010-03-20 21:15:40.483 Scrap[52367:207] CurrentCalendar Date: 2010-06-01 00:00:00 -0400
2010-03-20 21:15:40.483 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 6
2010-03-20 21:15:40.484 Scrap[52367:207] GregorianCalendar Date: 2010-06-01 00:00:00 -0400
2010-03-20 21:15:40.485 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 6
2010-03-20 21:15:40.485 Scrap[52367:207] CurrentCalendar Date: 2010-07-01 00:00:00 -0400
2010-03-20 21:15:40.486 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 7
2010-03-20 21:15:40.486 Scrap[52367:207] GregorianCalendar Date: 2010-07-01 00:00:00 -0400
2010-03-20 21:15:40.487 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 7
2010-03-20 21:15:40.488 Scrap[52367:207] CurrentCalendar Date: 2010-08-01 00:00:00 -0400
2010-03-20 21:15:40.488 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 8
2010-03-20 21:15:40.489 Scrap[52367:207] GregorianCalendar Date: 2010-08-01 00:00:00 -0400
2010-03-20 21:15:40.489 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 8
2010-03-20 21:15:40.490 Scrap[52367:207] CurrentCalendar Date: 2010-09-01 00:00:00 -0400
2010-03-20 21:15:40.491 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 9
2010-03-20 21:15:40.491 Scrap[52367:207] GregorianCalendar Date: 2010-09-01 00:00:00 -0400
2010-03-20 21:15:40.492 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 9
2010-03-20 21:15:40.493 Scrap[52367:207] CurrentCalendar Date: 2010-10-01 00:00:00 -0400
2010-03-20 21:15:40.493 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 10
2010-03-20 21:15:40.494 Scrap[52367:207] GregorianCalendar Date: 2010-10-01 00:00:00 -0400
2010-03-20 21:15:40.494 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 10
2010-03-20 21:15:40.495 Scrap[52367:207] CurrentCalendar Date: 2010-11-01 00:00:00 -0400
2010-03-20 21:15:40.496 Scrap[52367:207] CurrentCalendar: 5 Weeks in Month 11
2010-03-20 21:15:40.496 Scrap[52367:207] GregorianCalendar Date: 2010-11-01 00:00:00 -0400
2010-03-20 21:15:40.497 Scrap[52367:207] GregorianCalendar: 5 Weeks in Month 11
2010-03-20 21:15:40.498 Scrap[52367:207] CurrentCalendar Date: 2010-12-01 00:00:00 -0500
2010-03-20 21:15:40.498 Scrap[52367:207] CurrentCalendar: 52 Weeks in Month 12
2010-03-20 21:15:40.499 Scrap[52367:207] GregorianCalendar Date: 2010-12-01 00:00:00 -0500
2010-03-20 21:15:40.500 Scrap[52367:207] GregorianCalendar: 52 Weeks in Month 12

Compiling the code for Cocoa just for kicks, was actually amusing... As the results are really really different

2010-03-20 21:11:24.610 ScrapCocoa[52313:a0f] Calendar with 'currentCalendar' Identifier: gregorian
2010-03-20 21:11:24.611 ScrapCocoa[52313:a0f] Calendar created with identifier Identifier: gregorian
2010-03-20 21:11:24.613 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-01-01 00:00:00 -0500
2010-03-20 21:11:24.613 ScrapCocoa[52313:a0f] CurrentCalendar: 6 Weeks in Month 1
2010-03-20 21:11:24.614 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-01-01 00:00:00 -0500
2010-03-20 21:11:24.615 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 1
2010-03-20 21:11:24.616 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-02-01 00:00:00 -0500
2010-03-20 21:11:24.616 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 2
2010-03-20 21:11:24.617 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-02-01 00:00:00 -0500
2010-03-20 21:11:24.618 ScrapCocoa[52313:a0f] GregorianCalendar: 4 Weeks in Month 2
2010-03-20 21:11:24.619 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-03-01 00:00:00 -0500
2010-03-20 21:11:24.619 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 3
2010-03-20 21:11:24.620 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-03-01 00:00:00 -0500
2010-03-20 21:11:24.621 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 3
2010-03-20 21:11:24.622 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-04-01 00:00:00 -0400
2010-03-20 21:11:24.622 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 4
2010-03-20 21:11:24.623 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-04-01 00:00:00 -0400
2010-03-20 21:11:24.623 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 4
2010-03-20 21:11:24.624 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-05-01 00:00:00 -0400
2010-03-20 21:11:24.625 ScrapCocoa[52313:a0f] CurrentCalendar: 6 Weeks in Month 5
2010-03-20 21:11:24.625 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-05-01 00:00:00 -0400
2010-03-20 21:11:24.626 ScrapCocoa[52313:a0f] GregorianCalendar: 6 Weeks in Month 5
2010-03-20 21:11:24.627 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-06-01 00:00:00 -0400
2010-03-20 21:11:24.627 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 6
2010-03-20 21:11:24.628 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-06-01 00:00:00 -0400
2010-03-20 21:11:24.628 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 6
2010-03-20 21:11:24.629 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-07-01 00:00:00 -0400
2010-03-20 21:11:24.630 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 7
2010-03-20 21:11:24.630 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-07-01 00:00:00 -0400
2010-03-20 21:11:24.631 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 7
2010-03-20 21:11:24.632 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-08-01 00:00:00 -0400
2010-03-20 21:11:24.632 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 8
2010-03-20 21:11:24.633 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-08-01 00:00:00 -0400
2010-03-20 21:11:24.633 ScrapCocoa[52313:a0f] GregorianCalendar: 6 Weeks in Month 8
2010-03-20 21:11:24.634 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-09-01 00:00:00 -0400
2010-03-20 21:11:24.635 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 9
2010-03-20 21:11:24.636 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-09-01 00:00:00 -0400
2010-03-20 21:11:24.636 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 9
2010-03-20 21:11:24.637 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-10-01 00:00:00 -0400
2010-03-20 21:11:24.637 ScrapCocoa[52313:a0f] CurrentCalendar: 6 Weeks in Month 10
2010-03-20 21:11:24.638 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-10-01 00:00:00 -0400
2010-03-20 21:11:24.639 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 10
2010-03-20 21:11:24.640 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-11-01 00:00:00 -0400
2010-03-20 21:11:24.640 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 11
2010-03-20 21:11:24.641 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-11-01 00:00:00 -0400
2010-03-20 21:11:24.641 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 11
2010-03-20 21:11:24.642 ScrapCocoa[52313:a0f] CurrentCalendar Date: 2010-12-01 00:00:00 -0500
2010-03-20 21:11:24.642 ScrapCocoa[52313:a0f] CurrentCalendar: 5 Weeks in Month 12
2010-03-20 21:11:24.643 ScrapCocoa[52313:a0f] GregorianCalendar Date: 2010-12-01 00:00:00 -0500
2010-03-20 21:11:24.644 ScrapCocoa[52313:a0f] GregorianCalendar: 5 Weeks in Month 12

I think this is when I give up...

Answer 1

Tried this code with the 10.6 SDK and it appears to work.

2010-03-14 19:11:49.424 Untitled[61397:a0f] Date: 2010-01-01 00:00:00 -0500
2010-03-14 19:11:49.425 Untitled[61397:a0f] 6 Weeks in Month 1
2010-03-14 19:11:49.426 Untitled[61397:a0f] Date: 2010-02-01 00:00:00 -0500
2010-03-14 19:11:49.427 Untitled[61397:a0f] 5 Weeks in Month 2
2010-03-14 19:11:49.428 Untitled[61397:a0f] Date: 2010-03-01 00:00:00 -0500
2010-03-14 19:11:49.429 Untitled[61397:a0f] 5 Weeks in Month 3
2010-03-14 19:11:49.429 Untitled[61397:a0f] Date: 2010-04-01 00:00:00 -0400
2010-03-14 19:11:49.430 Untitled[61397:a0f] 5 Weeks in Month 4
2010-03-14 19:11:49.431 Untitled[61397:a0f] Date: 2010-05-01 00:00:00 -0400
2010-03-14 19:11:49.431 Untitled[61397:a0f] 6 Weeks in Month 5
2010-03-14 19:11:49.432 Untitled[61397:a0f] Date: 2010-06-01 00:00:00 -0400
2010-03-14 19:11:49.433 Untitled[61397:a0f] 5 Weeks in Month 6
2010-03-14 19:11:49.434 Untitled[61397:a0f] Date: 2010-07-01 00:00:00 -0400
2010-03-14 19:11:49.434 Untitled[61397:a0f] 5 Weeks in Month 7
2010-03-14 19:11:49.435 Untitled[61397:a0f] Date: 2010-08-01 00:00:00 -0400
2010-03-14 19:11:49.436 Untitled[61397:a0f] 5 Weeks in Month 8
2010-03-14 19:11:49.437 Untitled[61397:a0f] Date: 2010-09-01 00:00:00 -0400
2010-03-14 19:11:49.437 Untitled[61397:a0f] 5 Weeks in Month 9
2010-03-14 19:11:49.438 Untitled[61397:a0f] Date: 2010-10-01 00:00:00 -0400
2010-03-14 19:11:49.439 Untitled[61397:a0f] 6 Weeks in Month 10
2010-03-14 19:11:49.439 Untitled[61397:a0f] Date: 2010-11-01 00:00:00 -0400
2010-03-14 19:11:49.440 Untitled[61397:a0f] 5 Weeks in Month 11
2010-03-14 19:11:49.441 Untitled[61397:a0f] Date: 2010-12-01 00:00:00 -0500
2010-03-14 19:11:49.441 Untitled[61397:a0f] 5 Weeks in Month 12

I tried it with the iPhone SDK and I get the same output you do. It is unclear that the iPhone SDK is outputting the correct values for many of the other months as well. I agree, for example, with the 10.6 SDK answer that Jan. has 6 weeks.

I do not see anything obviously wrong with your code.

I might first suggest heading to http://bugreport.apple.com and submitting a bug.

You can also head over to Apple's private developer forums ( http://developer.apple.com/devforums/ ) and try asking there.

Of course, you can also submit a tech support incident to Apple ( [email protected] - http://developer.apple.com/programs/adcbenefits/ ) ... if you don't have any in your ADC account, I believe Apple will happily sell you one and you can speak directly with an Apple engineer about this issue.