Question

I’m trying to work with dates and create dates in the future, but daylight savings keeps getting in the way and messing up my times.

Here is my code to move to midnight of the first day of the next month for a date:

+ (NSDate *)firstDayOfNextMonthForDate:(NSDate*)date
{
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    calendar.timeZone = [NSTimeZone systemTimeZone];
    calendar.locale = [NSLocale currentLocale];

    NSDate *currentDate = [NSDate dateByAddingMonths:1 toDate:date];
    NSDateComponents *components = [calendar components:NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit
                                                    fromDate:currentDate];

    [components setDay:1];
    [components setHour:0];
    [components setMinute:0];
    [components setSecond:0];

    return [calendar dateFromComponents:components];
}

+ (NSDate *) dateByAddingMonths: (NSInteger) monthsToAdd toDate:(NSDate*)date
{
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    calendar.timeZone = [NSTimeZone systemTimeZone];
    calendar.locale = [NSLocale currentLocale];

    NSDateComponents * months = [[NSDateComponents alloc] init];
    [months setMonth: monthsToAdd];

    return [calendar dateByAddingComponents: months toDate: date options: 0];
}

Which give the dates when I run the method iteratively on a date:

2013-02-01 00:00:00 +0000
2013-03-01 00:00:00 +0000
2013-03-31 23:00:00 +0000 should be 2013-04-01 00:00:00 +0000
2013-04-30 23:00:00 +0000 should be 2013-05-01 00:00:00 +0000

My initial thought was to not use systemTimeZone but that didn’t seem to make a difference. Any ideas for how I can make the time constant and not take into account the change in daylight savings?

Answer 1

For a given calendar date/time, it is not possible as a general rule to predict what actual time (seconds since the epoch) that represents. Time zones change and DST rules change. It’s a fact of life. DST has a tortured history in Australia. DST rules have been very unpredictable in Israel. DST rules recently changed in the US causing huge headaches for Microsoft who was storing seconds rather than calendar dates.

Never save NSDate when you mean NSDateComponents. If you mean “the first of May 2013 in London,” then save “the first of May 2013 in London” in your database. Then calculate an NSDate off of that as close to the actual event as possible. Do all your calendar math using NSDateComponents if you care about calendar things (like months). Only do NSDate math if you really only care about seconds.

EDIT: For lots of very useful background, see the Date and Time Programming Guide.

And one more side note about calendar components: when I say “the first of May 2013 in London,” that does not mean “midnight on the first of May.” Don’t go adding calendar components you don’t actually mean.