Here is the problem: your app works with dates and you would like to be able to find out what day came a week before, or is coming up in two years. There are a few ways to do this but the easiest by far is to use your user’s calendar along with the NSDateComponents Foundation class to figure out precisely what these date relationships are.
Get Today’s Date In Objective-C
Before we go any further, let’s make sure we are on the same page and can get a reference to today’s date. Oh yeah, we’ll call that day today.
NSDate *today = [NSDate date];
What you can do is create a new NSDateComponents object and then specify the amount of time that you would like to add or subtract from your date.
NSDate *today = [NSDate date];
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
dateComponents.week = -1;
dateComponents.day = -3;
Now to get the date for one week and three days ago I need to get a reference to my system calendar first:
NSCalendar *calendar = [NSCalendar currentCalendar];
Then I can use the NSCalendar function dateByAddingComponents:toDate:options: to get the date from a week and three days ago.
NSDate *today = [NSDate date];
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
dateComponents.week = -1;
dateComponents.day = -3;
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *previousDate = [calendar dateByAddingComponents:dateComponents
toDate:today
options:0];
To see the results of the subtraction above write out the two date objects to the log.
NSDate *today = [NSDate date];
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
dateComponents.week = -1;
dateComponents.day = -3;
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *previousDate = [calendar dateByAddingComponents:dateComponents
toDate:today
options:0];
NSLog(@"Today is %@ and a week and three days ago it was %@", today, previousDate);
You will see something like this appear in your console screen:
Today is 2012-01-19 15:10:12 +0000 and a week and three days ago it was 2012-01-09 15:10:12 +0000
So What Do You Think?
Have you had any luck with this method of adding and subtracting dates in Objective-C?
Hi Matt
Would like to know what the performance of this approach would be. My app does a lot of date calculations and I am looking for something that performs really well.
Regards
@day2day – I don’t have any number on performance. The only thing I can say to improve performance is try caching some objects if needed. You will have to test to see if this is sufficient for your app. The alternative is to use NSTimeInterval and do your own calculations.
OK, now that you covered the basics, how about letting the iOS newbies in on the way it’s done in the REAL world: search github for a good date math library and forget you ever heard of NSDateComponents.
@Mark – did you have anything in particular in mind? For me, I rather not bring in third party libraries for these sorts of minor tasks that are already covered by the framework. But, there are plenty of ways to get things done so whatever floats your boat.