Format date to secounds - ios

Question

Background: I have dates stored in files.
What I would like to do: I would like to take the difference between two dates in seconds. I can't find any way to do it. My date format looks like that:
2015-23-02-12-23-43

Answer 1

Try this out:
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"yyyy-mm-dd-hh-mm-ss"];
NSDate *date = [dateFormatter dateFromString:string1];
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components:(NSCalendarUnitHour | NSCalendarUnitMinute) fromDate:date];
NSInteger hour = [components hour];
NSInteger minute = [components minute];

Answer 2

Try this:
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc]init];
[dateFormatter setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
NSDate *date = [dateFormatter dateFromString:EnterYourStringHere];
NSString *str = [dateFormatter stringFromDate:date];

Answer 3

If all you need is seconds then use
NSTimeInterval dateInterval = [date timeIntervalSince1970];
Then
NSString secondsString = [NSString stringWithFormat: #"%.0f", dateInterval];
EDIT:
If you have a file full of date strings that look like your example, 2015-23-02-12-23, then you could use code like this:
NSDateFormatter *myFormatter = [[NSDateFormatter alloc] init];
[myFormatter setDateFormat:#"yyyy-mm-dd-hh-mm-ss"];
//an example - you'll read from your file
NSString *aDateString = #"2015-23-02-12-23";
NSDate aDate = [myFormatter dateFromString: aDateString];
NSTimeInterval dateSeconds = [aDate timeIntervalSince1970];
That will give you dateSeconds as a double precision floating point number, which is the norm for numeric date calculations since it deals with fractions of seconds. You can then do numeric comparisons of date's time intervals.
Note that most UNIX systems use the numeric values returned by the timeIntervalSince1970 method, but Mac and iOS uses numeric date values returned by the method timeIntervalSinceReferenceDate. The two methods have different "epoch dates", or dates where they start counting from zero.
timeIntervalSince1970 uses midnight on Jan 1, 1970 (GMT)
timeIntervalSinceReferenceDate uses midnight on Jan 1, 2001 (GMT)
Which you use is up to you, just be sure to use the same method in all cases.
By the way, your date string format is horrible. having all those 2 digit numbers separated by dashes doesn't give the reader any way to tell apart month/day/hours/minutes/seconds. It's all a jumble. You'd be much better off using a standard date format.
In the US it's common to display dates in the form mm/dd/yyyyy (or yyyy/mm/dd, or even yyyy/dd/mm), and times as hh:mm:ss, so in "mm/dd/yyyyy hh:mm:ss" format you'd get:"09/02/2015 13:09:39" (I'm using human-readable date format strings for discussion, not those intended to set up a date formatter.)