Question

I am using this method to convert a month and year to a date which equals the last day in the month of the year given.

+ (NSDate*)endOfMonthDateForMonth:(int)month year:(int)year
{
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    comps.year = year;
    comps.month = month;

    NSDate *monthYearDate = [calendar dateFromComponents:comps];

    // daysRange.length will contain the number of the last day of the endMonth:
    NSRange daysRange = [calendar rangeOfUnit:NSDayCalendarUnit inUnit:NSMonthCalendarUnit forDate:monthYearDate];
    comps.day = daysRange.length;
    comps.hour = 0;
    comps.minute = 0;
    comps.second = 0;
    [comps setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    [calendar setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    [calendar setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]];
    NSDate *endDate = [calendar dateFromComponents:comps];
    return endDate;
}

I want the date to have a time component of 00:00:00, thats why I have set the time zone to GMT 0 and the date components for minutes, hours and seconds to 0. The date returned from the method is correct and has a time component from 00:00:00.

This is how I save the date to Core Data:

NSDate *endDate = [IBEstPeriod endOfMonthDateForMonth:endMonth year:endCalYear];
[annualPeriod setEndDate:endDate];

After retrieving the data and NSLogging it to the debugger console, I get dates like 2008-12-30 23:00:00 +0000 with a time component != 0.

Why did the component change now?
Shouldn’t it stay at 00:00:00?

What did I code wrong here?

Thank you!!

Answer 1

You just need to set the calendar time zone after creating calendar.

Add this as the second line of your function:

calendar.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];

However, an easier way to do it would be this:

- (NSDate*)endOfMonthDateForMonth:(int)month year:(int)year
{
    NSCalendar *calendar    = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    calendar.timeZone       = [NSTimeZone timeZoneWithName:@"UTC"];

    NSDateComponents *comps = [[NSDateComponents alloc] init];
    comps.year              = year;
    comps.month             = month+1;
    comps.day               = 0;

    NSDate *monthYearDate   = [calendar dateFromComponents:comps];
    return monthYearDate;
}

Results:

NSLog(@"%@",[self endOfMonthDateForMonth:12 year:2010]);
// Output: 2012-07-10 12:30:05.999 Testing App[16310:fb03] 2010-12-31 00:00:00 +0000

This works by setting the date to the “0th” day of next month, which is the same as the last day of this month. (This does the same thing as subtracting one day from the first day of next month.) Note that this works because comps are allowed to “overflow” (or in this case “underflow”) and dateFromComponents: does the math automatically.