NSDateFormatter's dateFromString returning null with date from stringFromDate

Question

I have a crash trying to add a nil date in an NSDictionary.

My code looks like this:

625  NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
626  NSCalendar *currentCalendar = [NSCalendar autoupdatingCurrentCalendar];
627  NSDate *todayDate = [NSDate date];
628
629  NSDateComponents *dateComponents = [currentCalendar components:NSUIntegerMax fromDate:todayDate];
630
631  NSMutableArray *lastDates = [[NSMutableArray alloc] init];
632
633  dateFormatter.dateFormat = @"y-M-d";
634
635  for (NSInteger i = 0; i < 7; i++) {
636      NSDate *date = [currentCalendar dateFromComponents:dateComponents];
637
638      dateComponents.day--;
639
640      [lastDates addObject:[dateFormatter stringFromDate:date]];
641  }
642
643  NSString *query = @"SELECT date FROM table";
644  NSArray *completedDates = [ZeeSQLiteHelper readQueryFromDB:query];
645
646  NSMutableDictionary *completedLessons = [[NSMutableDictionary alloc] init];
647
648  for (NSString *date in lastDates) {
649      BOOL completed;
650
651      if ([completedDates containsObject:@{ @"date" : date }]) {
652          completed = YES;
653   } else {
654       completed = NO;
655   }
656
657   completedLessons[[dateFormatter dateFromString:date]] = @(YES); // crash
658  }

For some reason, sometimes, dateFromString: returns nil and it crashes.

Crash report:

0     CoreFoundation      0    x231d068b    __exceptionPreprocess + 124
1     libobjc.A.dylib     0    x3458ae17    objc_exception_throw + 36
2     CoreFoundation      0    x230ec8fb    -[__NSDictionaryM setObject:forKey:] + 840
3     MyApp               0    x00148df7    -[MyClass myMethod] (MyClass.m:657)
4     MyApp               0    x001406f3    -[MySecondViewController mySecondVCMethod] (MySecondViewController.m:250)
5     MyApp               0    x0013f8ed    -[MySecondViewController viewDidLoad] (MySecondViewController.m:92)
6     UIKit               0    x2729443d    -[UIViewController loadViewIfRequired] + 1106
7     UIKit               0    x27293fcd    -[UIViewController view] + 22
8     UIKit               0    x27abb559    -[_UIFullscreenPresentationController _setPresentedViewController:] + 70
9     UIKit               0    x275c9339    -[UIPresentationController initWithPresentedViewController:presentingViewController:] + 110
10    UIKit               0    x275ec473    -[UIViewController _presentViewController:withAnimationController:completion:] + 3536
11    UIKit               0    x275eeac5    __62-[UIViewController presentViewController:animated:completion:]_block_invoke + 334
12    UIKit               0    x275eed31    -[UIViewController _performCoordinatedPresentOrDismiss:animated:] + 414
13    UIKit               0    x27388701    -[UIViewController presentViewController:animated:completion:] + 142
14    MyApp               0    x00223429    -[MyFirstViewController myFirstVCSecondMethod] (MyFirstViewController.m:775)
15    MyApp               0    x00221edb    -[MyFirstViewController myFirstVCFirstMethod:] (MyFirstViewController.m:631)
16    UIKit               0    x272ca795    -[UIApplication sendAction:to:from:forEvent:] + 78
17    UIKit               0    x272ca725    -[UIControl sendAction:to:forEvent:] + 62
18    UIKit               0    x272b354b    -[UIControl _sendActionsForEvents:withEvent:] + 444
19    UIKit               0    x272ca085    -[UIControl touchesEnded:withEvent:] + 602
20    UIKit               0    x27286d33    _UIGestureRecognizerUpdate + 10768
21    UIKit               0    x272c36f9    -[UIWindow _sendGesturesForEvent:] + 902
22    UIKit               0    x272c2e43    -[UIWindow sendEvent:] + 620
23    UIKit               0    x272947e5    -[UIApplication sendEvent:] + 202
24    UIKit               0    x27292fdf    _UIApplicationHandleEventQueue + 4932
25    CoreFoundation      0    x23193c3f    __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 12
26    CoreFoundation      0    x2319382d    __CFRunLoopDoSources0 + 450
27    CoreFoundation      0    x23191b9b    __CFRunLoopRun + 792
28    CoreFoundation      0    x230e5249    CFRunLoopRunSpecific + 518
29    CoreFoundation      0    x230e5035    CFRunLoopRunInMode + 106
30    GraphicsServices    0    x2c1c8ad1    GSEventRunModal + 158
31    UIKit               0    x272fa899    UIApplicationMain + 142
32    MyApp               0    x00152f83    main (main.m:16)
33    libdyld.dylib       0    x34cd6873    start + 0

Also, I am 99% sure that this occurs in Brazil, on 18 October, where Daylight Saving Time occured.

Try printing your date string and verify if it's format is same as format you using i.e. y-M-d — rptwsthi, Oct 27 '15 at 08:25
I can't, I get there crash reports through `Crittercism`. I couldn't reproduce it myself. But from the looks, having no external factors, it should be ok. — Iulian Onofrei, Oct 27 '15 at 08:26
For workaround, you could check the date is not null before adding in to dictionary.. — Vijay, Oct 27 '15 at 08:29
I guess `dateComponents.day` == 0 (from `dateComponents.day--;`) can cause the error. — tuledev, Oct 27 '15 at 08:30
@Vijay, I know I can do this, but I would want to know what caused this problem. And solve at it's core. — Iulian Onofrei, Oct 27 '15 at 08:30
Try to set `dateComponents.day = 1;dateComponents.day--;`. And what happen? — tuledev, Oct 27 '15 at 08:33
Why aren't you storing the _dates_ in your array? Formatting them as strings and then parsing those strings to get a date back again is expensive and, as you can see, error prone. (Also, please mark line 657 for us.) — DarkDust, Oct 27 '15 at 08:42
@DarkDust, It's because I need the date string to make some comparisons, but yes, it would be less expensive, though I don't agree to your comment's last part. It shouldn't be error prone. — Iulian Onofrei, Oct 27 '15 at 08:47
@IulianOnofrei: Depending on the date format, you're losing information. For example, with your date format `y-M-d`, you're losing the time and time zone information. — DarkDust, Oct 27 '15 at 08:51
@DarkDust, I don't need the time. In my database, I have a `STRING` column named `date` which contains dates in that format (e.g. 2-15-10-18). — Iulian Onofrei, Oct 27 '15 at 09:12

score 1 · Accepted Answer · edited May 23 '17 at 12:00

1

This answer and @DarkDust's comment pointed me in the right direction.

It was indeed a problem with the Daylight Saving Time occuring at midnight in Brazil. So between 17 and 18 October, there was no midnight. So because I had the date format set to y-M-d, dateFromString: was trying to return a date at the local midnight, which does not exist, hence it returned nil.

Steps to reproduce:

dateFormatter.timeZone = [NSTimeZone timeZoneWithName:@"America/Sao_Paulo"];
NSLog([dateFormatter dateFromString:@"2015-10-18"]); // returns nil

I still have to find out a solution that would return me a date like 2015-10-18 01:00:00 for this time zone.

edited May 23 '17 at 12:00

Community

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answered Oct 27 '15 at 09:45

Iulian Onofrei

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Ah, Jon's answer is a classic I forgot about… Since you can detect the error either due to the `nil` return or by using [`getObjectValue:forString:range:error:` which returns an error in the NSCocoaErrorDomain](https://gist.github.com/DarkDust/5fbfafb3c9350b38666a) you could modify the string by appending a time and falling back to a different date formatter which includes the time. You might also want to experiment with [CFDateFormatter](https://developer.apple.com/library/mac/documentation/CoreFoundation/Reference/CFDateFormatterRef/): maybe it has the same issue, maybe not. – DarkDust Oct 27 '15 at 11:22
@DarkDust, I could do something like `dateComponents.hour = 8`, but will it solve it in 100% times? – Iulian Onofrei Oct 27 '15 at 11:25
I guess you can't be sure. We could look into the timezone infos today and might determine that yes, it would work every time. But what about the next time [Kim Jong-Un smokes too much pot](https://en.wikipedia.org/wiki/Time_in_North_Korea) or some other country comes up with something stupid? Since it's a fallback for a seldom occurring issue you might be reasonably safe with such a workaround. But probably the best way to solve this is to eliminate the call to `dateFromString:` altogether by storing both the string and date in your first loop. – DarkDust Oct 27 '15 at 12:42

NSDateFormatter's dateFromString returning null with date from stringFromDate

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