How does one subtract hours from an NSDate?

Question

I would like to subtract 4 hours from a date. I read the date string into an NSDate object use the following code:

NSDateFormatter * dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
[dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZ"];
NSDate * mydate = [dateFormatter dateFromString:[dict objectForKey:@"published"]];

What do I do next?

Answer 1

NSCalendar is the general API for changing dates based on human time units. For this, you can use NSCalendar's -dateByAddingComponents:toDate:options: with a negative number of hours.

Answer 2

NSDate *newDate = [theDate addTimeInterval:-3600*4];

Link to documentation.

Strange that this method returns a new NSDate object, it completely differs from the naming conventions. I wonder if the returned date is autoreleased, probably yes.

---

NSDate *newDate = [[[NSDate alloc] initWithTimeInterval:-3600*4
                                              sinceDate:theDate]] autorelease];

Link to documentation.

Answer 3

Still not working.... what am I doing wrong?

NSDateFormatter * dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
[dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZ"];
NSString * datetemp = [dict objectForKey:@"published"];
NSString * theDate = [dateFormatter dateFromString:datetemp];
NSDate *newDate = [theDate addTimeInterval:-3600*4];
self.dateLabel.text = newDate;

Answer 4

Hey Miriam,
dateFromString function returns NSDate, not NSString. you should change,

NSDate * theDate = [dateFormatter dateFromString:datetemp];

hope it helps,
bests.
G.