Converting NSDate - objective-c

Question

I have an NSDate (lets call it x), 12 September, 2012 10:18PM (GMT). I want to convert x to a minute before my current time zone's (EST) midnight. So, x represented in EST with NSDateFormatter after conversion would be 12 September, 2012 11:59PM (EST). What's the best way to do this?
Thanks

Answer 1

Take a look at NSDateComponents: https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSDateComponents_Class/Reference/Reference.html
I believe you'll need to convert the NSDate to NSDateComponents, set the time to 11:59PM, then convert back to NSDate.
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [calendar components:(NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit) fromDate:date];
[components setHour:23];
[components setMinute:59];
NSDate *convertedDate = [calendar dateFromComponents:components];
NSLog(#"date=%#, convertedDate=%#", date, convertedDate);

Answer 2

Such manipulations can be easily done with the numeric representation of a date. In this representation, you deal with the number of seconds since a reference date.
The reference date for the purpose of the timeIntervalSinceReferenceDate is January 1st, 2001, at 00:00:00 GMT.
NSDate* date = [NSDate date];
NSInteger secondsSinceReferenceDate = [date timeIntervalSinceReferenceDate];
secondsSinceReferenceDate += 86400 - (secondsSinceReferenceDate % 86400);
secondsSinceReferenceDate -= 60;
secondsSinceReferenceDate -= [NSTimeZone.localTimeZone secondsFromGMTForDate:date];
NSDate* justBeforeToday =
[NSDate dateWithTimeIntervalSinceReferenceDate:secondsSinceReferenceDate];
NSLog(#"Date used was %#", date);
NSLog(#"Just before tomorrow is %#", justBeforeToday);
Since there are 86400 seconds in a day (24 hours times 60 minutes 60 times 60 seconds = 86400 seconds), you know that 86400 - (secondsSinceReferenceDate % 86400) is the number of seconds there are still to midnight. So if you take today's date (or any other valid date), add this number of seconds, and then subtract another 60 seconds, you'll have today's evening at 11:59 PM in the GMT timezone.
With [NSTimeZone.localTimeZone secondsFromGMTForDate:], you know how many seconds your timezone is offset from the GMT timezone. By subtracting this offset to your integer representation, you effectively get when it will be 11:59 PM in your local timezone.
Here's a sample output:
Date used was 2012-09-12 22:37:49 +0000
Just before tomorrow is 2012-09-13 03:59:00 +0000
I'm in the EDT timezone too, and this looks like the correct answer (remember Standard Time is -5 from GMT, but right now we're in daylight savings, so it's -4 from GMT, which is invariant).