Calculating the end of a month, or its last second

I have the simplest of tasks that I can't figure out how to do correctly. With the snippet below I can find out the beginning of a month.

NSCalendar *cal = [NSCalendar currentCalendar];
NSDate *beginning = nil;
[cal rang开发者_如何学PythoneOfUnit:NSMonthCalendarUnit startDate:&beginning interval:NULL forDate:self];
return beginning;

Now I want to determine the end of that same month (the last second at 23:59:59 would suit my purposes). My first thought was to start at the first day of the next month and subtract a second. But I couldn't break down the date in an NSDateComponent instance to [dateComponents setMonth:[dateComponents month] + 1], because in the case of December that method wouldn't accept 12 + 1 = 13 to get to January, right?

Then how would I get to the last second of a month?

Do this to get the beginning of next month:

NSDateComponents *comps = [[NSDateComponents alloc] init];
[comps setMonth:1];
NSDate *beginningOfNextMonth = [cal dateByAddingComponents:comps toDate:beginning options:0];
[comps release];

Alright, I think I have it. I'm using the rangeOfUnit:inUnit:forDate: method to determine the length of the current month in days, and add the corresponding NSTimeInterval to the month's start date.

- (NSDate *)firstSecondOfTheMonth {
    NSCalendar *cal = [NSCalendar currentCalendar];
    NSDate *beginning = nil;
    if ([cal rangeOfUnit:NSMonthCalendarUnit startDate:&beginning interval:NULL forDate:self])
        return beginning;
    return nil;
}

- (NSDate *)lastSecondOfTheMonth {
    NSDate *date = [self firstSecondOfTheMonth];
    NSCalendar *cal = [NSCalendar currentCalendar];
    NSRange month = [cal rangeOfUnit:NSDayCalendarUnit inUnit:NSMonthCalendarUnit forDate:date];
    return [date dateByAddingTimeInterval:month.length * 24 * 60 * 60 - 1];
}

Note: Although this code works as I intended it to work, the answer that @spacehunt gave more clearly communicates the purpose.

The last second of the month... Most of the time, that boils down to taking the first second of the next month, and subtracting one second.

But are you asking the right question? What's the meaning of that last second? My guess is that you want to express a time interval starting from (and including) the first second of month N, and ending right before the first second of month N+1.