NSDate returning different value on conversion from string - Not GMT related

Question

I hope this isn't GMT related or I will feel rather stupid.

2 quick related questions. Why is this converting to a different date? Is losing a day. I looked at time zones as stated in other answers but it is always the GMT timezone 0000 which is what I expected. I presume the error is in my setDateFormat but I can't see how to fix it.

NSString *stringFromDate = self.dateLabel.currentTitle;
NSLog(@"StringFromDateWeight! %@", stringFromDate);

NSDateFormatter *df = [[NSDateFormatter alloc] init];
//Convert Back to NSDate
[df setDateFormat:@"ddMMyyyy"];

NSDate *inputedDate = [df dateFromString: stringFromDate];
NSLog(@"StringFromDateWeight2! %@", inputedDate);

NSLog(@"StringFromDateWeight! %@", stringFromDate); is
17072013

NSLog(@"StringFromDateWeight2! %@", inputedDate); is

2013-07-16 23:00:00 +0000

I am also using the code below to compare 2 dates and am I right in that it returns in seconds? How would I change it to return in days?

int intervall = (int) [theDate timeIntervalSinceDate: now];

It is indeed a timezone issue. When you convert the string to NSDate the dateFormatter uses your local timezone. Which appears to be UTC+1 and "midnight July 17" in your local timezone is "July 16 23:00:00" in UTC. If you want to calculate in UTC you have to set the timezone of the dateFormatter `df.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];` If you don't want to do this don't trust NSLog'd NSDates, use `[inputedDate descriptionWithLocale:[NSLocale currentLocale]]`. It's usually better to not look at the UTC date at all. It confuses stackoverflow users every day. ;-) — Matthias Bauch, Jul 17 '13 at 20:48
@MatthiasBauch thank you very much that fixed it. I upvoted u as it is very close to a answer tbh. Any thoughts on the final bit? — memyselfandmyiphone, Jul 17 '13 at 20:59

score 4 · Accepted Answer · edited May 23 '17 at 12:05

If you don't explicitly set a timezone NSDateFormatter will use your local timezone. You don't set one, so your formatter will create a NSDate that is at "midnight July 17" in your timezone. The description method of NSDate will return a date that is formatted in UTC timezone. Since you get "July 16 23:00:00" I guess your timezone is UTC+1.

You have two options. Calculate in UTC by setting the timezone explicitly.

df.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];

Or, usually more useful, don't look at the description of NSDate and use [inputedDate descriptionWithLocale:[NSLocale currentLocale]] for debugging, which will print the date formatted for your timezone.
If you want to display the date to the user use another NSDateFormatter (preferably with dateStyle and timeStyle and not dateFormat, because hardcoded dateFormats are evil)

It's just the display that is different, the underlying NSDate object is still the same.

Regarding your second question:

In many timezones there are 2 days each year that don't have 24 hours, so you can't calculate anything with the seconds you get from timeIntervalSinceDate:.

You have to use NSDateComponents and NSCalendar. Fortunately there is already a method that does exactly what you want. components:fromDate:toDate:options:

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [calendar components:NSDayCalendarUnit|NSHourCalendarUnit|NSMinuteCalendarUnit|NSSecondCalendarUnit fromDate:inputedDate toDate:[NSDate date] options:0];
NSLog(@"Date was %d days (and %d hours, %d minutes and %d seconds) ago", components.day, components.hour, components.minute, components.second);

If you only need the number of days you can remove all components except NSDayCalendarUnit

NSDate returning different value on conversion from string - Not GMT related

1 Answers1