Why is NSFormatter dateFromString occasionally returning nil?

Question

I have a method to convert a string in a specific format into an NSDate but it occasionally returns nil.

- (NSDate *)dateFromString:(NSString *)dateAndTime
{
    NSDate *date;

    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:@"MM/dd/yyyy hh:mm a"];

    NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
    [formatter setTimeZone:gmt];

    date = [formatter dateFromString:dateAndTime];

    return date;
}

This works most of the time from what I can tell, but there's a percentage of time that it fails.

I've been tracking this bug for a while so there is some code that makes the app crash when this happens. The crash reports tell me the values that made the method return nil.

For example, I received a crash report telling me that "4/14/2015 7:35 PM" returned nil.

When I hard code the same value for every date, it works when running the app in debug mode from my computer.

What could be going on here? Am I not covering some cases for NSDates? How come it doesn't happen on every device?

"4/14/2014" does not match "MM/dd/yyyy". "04/14/2014" would match. You also need to set the locale to "en_US_POSIX". The need for that is covered in many discussions here and in Apple's documentation. — rmaddy, Apr 15 '15 at 00:04
http://stackoverflow.com/questions/6613110/what-is-the-best-way-to-deal-with-the-nsdateformatter-locale-feature — Hot Licks, Apr 15 '15 at 00:25

Joey · Accepted Answer · 2015-04-15T05:39:24.057

2

First, the date string of 4/14/2015 7:35 PM is possibly M/d/yyyy h:m a or M/dd/yyyy h:mm a. http://www.unicode.org/reports/tr35/tr35-25.html#Date_Format_Patterns

Second, it is recommended to set NSLocale to NSDateFormatter.

formatter.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];

I had a same issue that NSDate was set nil when I worked with a Chinese guy. It happened only on the specific devices, and the problem was region settings.

To reproduce the problem, follow below steps:

go to Settings app
General -> Language & Region -> Advanced
Turn off Automatic
change Language to Chinese, Simplified
execute your app again. You will find NSDate is always nil if NSLocale is NOT set.

edited Apr 15 '15 at 05:39

answered Apr 15 '15 at 00:43

Joey

2,912
2
27
32

1

The locale needs to be "en_US_POSIX", not just "en_US". – rmaddy Apr 15 '15 at 05:38
Yep - I was able to reproduce the issue with your suggestions. The only thing I'm not really clear on (even after the doc you posted) is the difference between M and MM. The app seems to be working now that I set the locale, regardless of using MM. – kraftydevil Apr 16 '15 at 03:34

Why is NSFormatter dateFromString occasionally returning nil?

1 Answers1