Question

I have a class that holds an start date and an end date, normally initialised to the firt and last second of the month.

The following function works correctly going from Nov 2010 forwards into December and back again however going backwards from November ends up with startDate set to

2010-09-30 23:00:00 GMT

Ie. a month and an hour ago.

Strangely the endDate is still correctly set to

2010-11-01 00:00:00 GMT

And going forward a month from this incorrect date also results in the correct time and date.

Is this a bug or am I doing something I shouldn’t be ?

-(void) moveMonth:(NSInteger)byAmount { // Positive or negative number of months
    NSCalendar *cal = [NSCalendar currentCalendar];

 NSDateComponents *components = [[[NSDateComponents alloc] init] autorelease];
 // Update the start date
 [components setMonth:byAmount];
 NSDate *newStartDate = [cal dateByAddingComponents:components toDate:[self startDate] options:0];
 [self setStartDate:newStartDate];

 // And the end date
 [components setMonth:1];
 NSDate *newEndDate = [cal dateByAddingComponents:components toDate:[self startDate] options:0 ];
 [self setEndDate:newEndDate];
}

SOLUTION: Answer correctly pointed out this is a DST issue

If you want to deal in absolute times and date then using the following avoids any DST being involved.

    NSCalendar *cal = [[NSCalendar alloc ] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
    NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"GMT"];
    [cal setTimeZone:zone];

Answer 1

It is probably not a bug but something related to DST changes in October-November period.