NSDateFormatter reports June 2, 2013 as being in w

I am using NSDateFormatter to convert a series of dates to a week number within a month.

The date formatting code I am using is yyyyMMW and everything I have read tells me that W will be between 1-5.

But, the 2nd of June 2013 fell on a Sunday (the default start day of the week in the gregorian calendar) and it's week number is reported as 0 even though the start date of the week is calculated correctly:

2013-06-03 14:15:45.611 date=20130531, week=2013055, start of week=20130526
2013-06-03 14:15:45.612 date=20130602, week=2013060, start of week=20130602
2013-06-03 14:15:45.612 date=20130603, week=2013061, start of week=20130602

Some quick and dirty test code to reproduce the log shown above:

    NSDateFormatter *dateFormatDaily = [[NSDateFormatter alloc] init];
[dateFormatDaily setDateFormat:@"yyyyMMdd"];
NSDateFormatter *dateFormatterWeekly = [[NSDateFormatter alloc] init];
[dateFormatterWeekly setDateFormat:@"yyyyMMW"];

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
[calendar setFirstWeekday:1]; // default but set here for clarity

NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
[dateComponents setMonth:5];
[dateComponents setDay:31];
[dateComponents setYear:2013];
NSDate *date_1 = [calendar dateFromComponents:dateComponents];
[dateComponents setMonth:6];
[dateComponents setDay:2];
NSDate *date_2 = [calendar dateFromComponents:dateComponents];
[dateComponents setDay:3];
NSDate *date_3 = [calendar dateFromComponents:dateComponents];
NSArray *datesToTest = @[date_1, date_2, date_3];

for (NSDate *date in datesToTest) {
    NSString *weekNo = [dateFormatterWeekly stringFromDate:date];
    NSDate *beginningOfWeek = nil;
    BOOL rc = [calendar rangeOfUnit:NSWeekCalendarUnit startDate:&beginningOfWeek interval:NULL forDate:date];
    if (rc) {
        NSLog(@"date=%@, week=%@, start of week=%@", [dateFormatDaily stringFromDate:date], weekNo, [dateFormatDaily stringFromDate:beginningOfWeek]);
    } else {
        NSLog(@"Could not calculate beginning of week");
    }
}

Any ideas? A week number of 0 under any circumstances seems wrong to me.

Thanks

There are various parameters that cause this effect. First of all, you did not set a calendar for the date formatter. If you add

[dateFormatterWeekly setCalendar:calendar];

to your code, then the output will be as you expected:

date=20130531, week=2013055, start of week=20130526
date=20130602, week=2013062, start of week=20130602
date=20130603, week=2013062, start of week=20130602

But in your case, the date formatter uses the current calendar, and therefore has separate parameters firstWeekDay and minimumDaysInFirstWeek. These parameters are locale dependent. If I test this on the iOS Simulator with the "Region Format" set to "German -> Germany", then

[[dateFormatterWeekly calendar] firstWeekday]           = 2
[[dateFormatterWeekly calendar] minimumDaysInFirstWeek] = 4

and I assume that you will have similar values, because now I get the same output as you.

Now for the date formatter, the week starts on a Monday, which means that June 2 is in the week starting at May 27. This counts as "week #0" in June, because only one day of this week is in June, but minimumDaysInFirstWeek = 4. The first week in a month that has at least minimumDaysInFirstWeek days, counts as "week #1".

(I found the relevance of the minimumDaysInFirstWeek parameter here: http://www.cocoabuilder.com/archive/cocoa/326845-week-of-month-confusion.html)