iOS: Date comparison failing under real-world conditions, works under simulated conditions

Question

I'm using this code to compare two dates, to see if the stored date is yesterday essentially:

NSDate *date = [wordData objectForKey:@"date"];
NSLog(@"Word data date: %@", [wordData objectForKey:@"date"]);

NSDate *nowDate = [NSDate date];
NSLog(@"Date now: %@", nowDate);

NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *date1Components = [cal components:NSCalendarUnitEra | NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:date];
NSDateComponents *date2Components = [cal components:NSCalendarUnitEra | NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:nowDate];
NSComparisonResult comparison = [[cal dateFromComponents:date1Components] compare:[cal dateFromComponents:date2Components]];


if(comparison == NSOrderedAscending)
{
    NSLog(@"Word out of date, fetching new date");
    [self fetchCurrentWord];
}
if(comparison == NSOrderedDescending)
{
    NSLog(@"Word is newer");
}
else if(comparison == NSOrderedSame)
{
    NSLog(@"Same date");
}

However, when the date changes on device normally the code is returning NSOrderedSame, if I simulate it by changing the device time manually it works, or if I use dateWithTimeIntervalSinceNow it works. I've added debug prints and the dates look fine, they're originally created using NSDate so there should be no format differences. I'm drawing a blank on what it could actually be.

Failure log:

Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Word is: CLIMATURE, A climate. [Obs.] Shak., 2014-10-16 23:05:16 +0000
Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Word CLIMATURE, definition A climate. [Obs.] Shak.
Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Word date: 2014-10-16 23:05:16 +0000
Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Word data date: 2014-10-16 23:05:16 +0000
Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Date now: 2014-10-17 10:25:05 +0000
Oct 17 11:25:05 Nicholas-Smiths-iPhone WOTD[2493] <Warning>: Same date

Success log:

2014-10-15 16:35:37.748 WOTD[960:31212] Word is: CONTORTUPLICATE, Plaited lengthwise and twisted in addition, as the bud of themorning-glory. Gray., 2014-10-15 14:43:04 +0000
2014-10-15 16:35:37.749 WOTD[960:31212] Word CONTORTUPLICATE, definition Plaited lengthwise and twisted in addition, as the bud of themorning-glory. Gray.
2014-10-15 16:35:37.749 WOTD[960:31212] Word date: 2014-10-15 14:43:04 +0000
2014-10-15 16:35:37.749 WOTD[960:31212] Word data date: 2014-10-15 14:43:04 +0000
2014-10-15 16:35:37.749 WOTD[960:31212] Date now: 2014-10-15 15:35:37 +0000
2014-10-15 16:35:37.750 WOTD[960:31212] Same date

I changed the time manually at this stage

2014-10-16 16:37:29.286 WOTD[1133:38254] Word is: CONTORTUPLICATE, Plaited lengthwise and twisted in addition, as the bud of themorning-glory. Gray., 2014-10-15 14:43:04 +0000
2014-10-16 16:37:29.287 WOTD[1133:38254] Word CONTORTUPLICATE, definition Plaited lengthwise and twisted in addition, as the bud of themorning-glory. Gray.
2014-10-16 16:37:29.287 WOTD[1133:38254] Word date: 2014-10-15 14:43:04 +0000
2014-10-16 16:37:29.287 WOTD[1133:38254] Word data date: 2014-10-15 14:43:04 +0000
2014-10-16 16:37:29.287 WOTD[1133:38254] Date now: 2014-10-16 15:37:29 +0000
2014-10-16 16:37:29.288 WOTD[1133:38254] Word out of date, fetching new date
2014-10-16 16:37:29.616 WOTD[1133:38538] Word PAC, definition A kind of moccasin, having the edges of the sole turned up andsewed to the upper. Knight.

Update your question with the log output so we can see what you are seeing. — rmaddy, Oct 15 '14 at 15:29
Sure, it'll have to wait until tomorrow for the log output (for obvious, hilariously frustrating reasons) of failure. I'll add one for success. — Nicholas Smith, Oct 15 '14 at 15:35
(And remember that the dates you're displaying are UTC, but you're comparing in your timezone. So the date may appear different due to the timezone difference, when it's not, or it may appear the same when it's not, also due to the timezone difference.) — Hot Licks, Oct 15 '14 at 15:46
It's currently BST, I didn't think about the knock on from UTC but the time difference should only be 1hr so when I have been checking (9am+) it should be next day for UTC too. — Nicholas Smith, Oct 15 '14 at 15:49

score 2 · Accepted Answer · answered Oct 17 '14 at 16:44

The example you're showing as failing actually is on the same day - this is down to comparing in UTC versus BST - the word date is just in the same day as 23:05 + 1hr = 00:05 = today.

You need to take care working with dates due to the timezones, if you want to know if something happened yesterday compared to now, a better approach might be:

NSDate *startOfDay;
NSTimeInterval lengthOfDayInSeconds;
BOOL success = [cal rangeOfUnit:NSCalendarUnitDay startDate:&startOfDay interval:&lengthOfDayInSeconds forDate:[NSDate date]];

This gives you a date for the start of today startOfDay (appropriate for the local calendar timezone), which you can then compare against your test date with a simple comparison.

Hi, I've implemented it (been busy with my real job), and once it's tested I'll accept the answer. Does look like it should do the job though, so thanks! — Nicholas Smith, Oct 22 '14 at 09:26

iOS: Date comparison failing under real-world conditions, works under simulated conditions

1 Answers1