Formatting Date Using NSDateFormatter Not Working - objective-c

Question

I have this NSString that has a date and time that I want to format:
Jul 17, 2013 09:10 PM
I want to format it like this:
yyyy-MM-dd HH:mm
but it doesn't work.
This is the code I'm using:
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[formatter setDateFormat:#"MMM dd, yyyy hh:mm a"];
NSDate *_date = [formatter dateFromString:_dateToFormat];
NSLog(#"_date: %#", _date);
[formatter setDateFormat:#"yyyy-MM-dd HH:mm"];
NSString *_newDate = [formatter stringFromDate:_date];
NSLog(#"_newDate: %#", _newDate);
NSLog returns:
_date: 2013-07-17 04:10:00 +0000
_newDate: 2013-07-17 04:10:00 +0000
Am I missing something? After formatting the date, I want to subtract 45 minutes from the formatted date using the code:
_newDate = [_date dateByAddingTimeInterval:-60*45];
NSLog(#"Subtracted date: %#", _newDate);
But this isn't my worry right now since the first step isn't working, which is to format the date from NSString. Any idea why it isn't working?
Edit
Updated the typo on the code for dateByAddingInterval by changing _newDate with _date.

Answer 1

Since your dateString is in AM/PM format you will have to use
[formatter setDateFormat:#"MMM dd, yyyy hh:mm a"];

Answer 2

you are using HH and a in your dateFormat. HH means "hour in 24 hour format", and it looks like it takes precedence over a, the period (i.e. PM).
Use #"MMM dd, yyyy hh:mm a" with hh, which means "hour in 12 hour format" to convert your string into a NSDate.
NSString *_dateToFormat = #"Jul 17, 2013 09:10 PM"; // this is in local time zone! (mine is UTC+2)
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[formatter setDateFormat:#"MMM dd, yyyy HH:mm a"];
NSDate *_date = [formatter dateFromString:_dateToFormat];
NSLog(#"wrong _date: %#", _date);
[formatter setDateFormat:#"MMM dd, yyyy hh:mm a"];
_date = [formatter dateFromString:_dateToFormat];
NSLog(#"correct _date: %#", _date); // this is in UTC, not in local time zone!
NSLog(#"correct _date: %#", [_date descriptionWithLocale:[NSLocale currentLocale]]); // this should be in your local timezone
Output:
wrong _date: 2013-07-17 10:10:00 +0000
correct _date: 2013-07-17 19:10:00 +0000 (in my timezone: 21:10, or 09:10 PM)
correct _date: Wednesday, July 17, 2013, 9:10:00 PM Central European Summer Time
Keep in mind that printing a NSDate usually prints in UTC. So if your timezone is different the logged NSDate will not match your input date. It will be off by the offset between your timezone and UTC.
You can print [_date descriptionWithLocale:[NSLocale currentLocale]] to see the time in your local timezone.
But this is just when you NSLog the NSDate. The NSDate is still correct, it's just the printed output that seems to be wrong.
To conclude, your code should look like this:
NSString *_dateToFormat = #"Jul 17, 2013 09:10 PM";
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[formatter setDateFormat:#"MMM dd, yyyy hh:mm a"];
// create date from string
NSDate *_date = [formatter dateFromString:_dateToFormat];
// subtract 45 minutes
_date = [_date dateByAddingTimeInterval:-60*45];
[formatter setDateFormat:#"yyyy-MM-dd HH:mm"];
// turn date into string
NSString *_newDate = [formatter stringFromDate:_date];
NSLog(#"%#", _newDate);
Output: 2013-07-17 20:25

Answer 3

Use 2 different formatters, one for converting the string to a date and a second for converting your date back to a string. The first should use the en_US_POSIX locale and the second should probably use the system (default) locale (but if you want to force a particular format then you can use en_US_POSIX here too).

Answer 4

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:#"MMM dd, yyyy HH:mm a"];
NSDate *_date = [formatter dateFromString:_dateToFormat];
NSlog(#"Date: %#",_date);

Answer 5

(Posted answer on behalf of the question author).
I managed to make it work using this code with the appropriate output logs:
NSString *oldDate = #"Jul 17, 2013 09:10 PM";
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[formatter setDateFormat:#"MMM dd, yyyy hh:mm a"];
NSDate *_date = [formatter dateFromString:oldDate];
NSLog(#"_date: %#", _date);
_date = [_date dateByAddingTimeInterval:-60*45];
NSLog(#"_date2: %#", _date);
[formatter setDateFormat:#"yyyy-MM-dd HH:mm"];
NSString *_newDate2 = [formatter stringFromDate:_date];
NSLog(#"_newDate2: %#", _newDate2);
Output:
_date: 2013-07-17 13:10:00 +0000
_newDate: 2013-07-17 21:10
_date2: 2013-07-17 12:25:00 +0000
_newDate2: 2013-07-17 20:25