Comparing dates issue in iOS

Question

I am using following code to compare two NSDates. The result is later used in a transient attribute to organise objects in table view sections. I can assure that there are enough objects in the core data store so that the three possible results are present, but the NSLog shows that option tmp = @"1", is never thrown.

-(NSString *)sectionIdentifier{

    [self willAccessValueForKey:@"sectionIdentifier"];
    NSString *tmp = [self primitiveValueForKey:@"sectionIdentifier"];
    [self didAccessValueForKey:@"sectionIdentifier"];

    if (!tmp){

        NSDate *date = self.todoDueDate;
        NSDate *todayDate = [NSDate date];

        NSLog(@"date= %@",date);
        NSLog(@"todayDate = %@",todayDate);



        if([date
            compare:todayDate] == NSOrderedSame) {
            tmp = @"1";//TODAY
        }
        else if([date
                 compare:todayDate] == NSOrderedDescending){
            tmp = @"2";//OVERDUE
        }
        else if ([date
                  compare:todayDate] == NSOrderedAscending){
            tmp =@"0";//UPCOMING
        }
        NSLog(@"Tmp= %@",tmp);

        [self setPrimitiveValue:tmp forKey:@"sectionIdentifier"];

    }

Here the log:

2014-01-15 10:10:52.732 To-Do Pro Light[21458:a0b] date      = 2014-01-10 19:53:19 +0000
2014-01-15 10:10:52.735 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.742 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.743 To-Do Pro Light[21458:a0b] date      = 2014-01-14 17:45:45 +0000
2014-01-15 10:10:52.746 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.747 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.748 To-Do Pro Light[21458:a0b] date      = 2014-01-14 17:57:44 +0000
2014-01-15 10:10:52.749 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.751 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.753 To-Do Pro Light[21458:a0b] date      = 2014-01-15 16:31:48 +0000
2014-01-15 10:10:52.755 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.756 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.758 To-Do Pro Light[21458:a0b] date      = 2014-01-15 16:32:06 +0000
2014-01-15 10:10:52.759 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.761 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.762 To-Do Pro Light[21458:a0b] date      = 2014-01-15 17:03:40 +0000
2014-01-15 10:10:52.763 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.764 To-Do Pro Light[21458:a0b] Tmp= 0
2014-01-15 10:10:52.766 To-Do Pro Light[21458:a0b] date      = 2014-01-15 18:33:48 +0000
2014-01-15 10:10:52.767 To-Do Pro Light[21458:a0b] todayDate = 2014-01-15 17:10:52 +0000
2014-01-15 10:10:52.768 To-Do Pro Light[21458:a0b] Tmp= 2

What am I doing wrong?

None of the `NSDate` objects in your log are the same. Why do you expect to see `Tmp= 1`? Remember, the time (to the millisecond) is included in the comparison. — rmaddy, Jan 15 '14 at 17:27
and how to compare dates without taking into account the time, only year, month and day? — mvasco, Jan 15 '14 at 17:29
You're never going to get an exact match, since two NSDate objects would have to refer to the same MILLISECOND in the day for that to occur. Simplest is to use an NSDateFormatter to format the dates to yyyy/MM/dd and compare strings. — Hot Licks, Jan 15 '14 at 17:34
@HotLicks, please put your comment in an answer to let me check it as the selected answer. I will change my code using NSDataFormatter. Thank you. — mvasco, Jan 15 '14 at 17:42

score 2 · Accepted Answer · answered Jan 15 '14 at 17:45

2

You're never going to get an exact match, since two NSDate objects would have to refer to the same MILLISECOND in the day for that to occur. Simplest is to use an NSDateFormatter to format the dates to yyyy/MM/dd and then compare the resulting strings.

(Don't forget to set timezone appropriately for your requirements.)

answered Jan 15 '14 at 17:45

Hot Licks

47,103
17
93
151

Using an `NSDateFormatter` is far less efficient than using date components. – rmaddy Jan 15 '14 at 17:51
I will edit my question to add what I have changed, now it is working..thank you.. – mvasco Jan 15 '14 at 17:52
@rmaddy - But much more straight-forward and foolproof. And the efficiency difference is not that great if the formatter is pre-created. – Hot Licks Jan 15 '14 at 17:54
@rmaddy - The pendulum would swing more towards using datecomponents if they supported Julian day, so that it wasn't necessary to retrieve and combine the three separate numbers. – Hot Licks Jan 15 '14 at 17:57
Yes that would be a nice addition but keep in mind that using a date formatter most likely uses date components behind the scenes. So now you throw on a bunch of string processing on top of that. That's a lot of wasted processing to save a line or two of code. – rmaddy Jan 15 '14 at 18:00
@rmaddy - Of course, if you're that concerned about efficiency you shouldn't be using Objective-C in the first place. But the point is that (especially with a novice) simpler is better, and the dateformatter approach is much simpler than the calendarcomponent approach. – Hot Licks Jan 15 '14 at 18:06
Are you sure it's simpler? It's hard to tell by the large number of date format related questions posted here every day. :) – rmaddy Jan 15 '14 at 18:07
@rmaddy - You got me there. The only other topic that comes close is JSON, and both are blazingly simple. But folks are always copying a "solution" from somewhere vs taking the ten minutes it takes to understand them. – Hot Licks Jan 15 '14 at 18:10

score 1 · Answer 2 · answered Jan 15 '14 at 17:52

NSDate *startDate = ...;
NSDate *endDate = ...;

NSCalendar *gregorian = [[NSCalendar alloc]
                 initWithCalendarIdentifier:NSGregorianCalendar];

NSUInteger unitFlags = NSMonthCalendarUnit | NSDayCalendarUnit | NSYearCalendarUnit;

NSDateComponents *components = [gregorian components:unitFlags
                                          fromDate:startDate
                                          toDate:endDate options:0];
NSInteger months = [components month];
NSInteger days = [components day];
NSInteger years = [components year];

If all three are 0 - the dates are equal.

More at - https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/DatesAndTimes/Articles/dtCalendricalCalculations.html#//apple_ref/doc/uid/TP40007836-SW1

Comparing dates issue in iOS

2 Answers2