iOS 6.1 NSDateFormatter has changed since iOS 5 - broken parsing? - iphone

Question

Our iOS iPhone app contains this code which produced a valid NSDate object below named resultDate in iOS 5:
static NSDateFormatter *invariantFmt = nil;
if (!invariantFmt) {
invariantFmt = [[NSDateFormatter alloc] init];
NSLocale *locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US_POSIX"];
[invariantFmt setLocale:locale];
[locale release];
[invariantFmt setDateStyle:NSDateFormatterShortStyle];
[invariantFmt setTimeStyle:NSDateFormatterMediumStyle];
}
NSDate *resultDate = [invariantFmt dateFromString:#"08/04/2010 10:43:39 AM"];
After upgrading to XCode 4.6 and iOS 6.1, that code now gives a nil for resultDate, so something has changed with what they use to parse. The release notes says nothing about NSDateFormatter changing. Internet research has turned up only that they might have changed to use a newer Unicode UTS Locale parsing standard. Obviously they changed something. After twiddling with code and taking a known valid NSDate object and applying the same NSDateFormatter settings to get a NSString, I find that iOS 6.1 likes this string instead: #"08/04/2010, 10:43:39 AM"
The only difference being that extra comma after the date portion. Using that in iOS 6.1 gives back a valid date with the same above code. Anyone seeing this and understand why that's different or if that's an ok Unicode change or a Apple bug?

Answer 1

The format styles should only be used to convert NSDate objects to text to display to the user. When parsing a date string in a known format, you must use a specific format, never the styles. The use of the en_US_POSIX locale is used to ensure that the format you specify isn't tweaked by the OS based on user preferences such as the 24-hour time setting.
So, as you suspected, you need to remove the two calls to set the date and time styles and replace them with a call to set a specific format that matches your known date/time string you need to parse.

Answer 2

Generally you should not output a formatted Date to a file, and later want to parse it back.
A Date should be stored as long value UTC (with or without additional TimeZone offset).
Only in the last moment, before visualizing a Date to UI it should be formatted and local time applied.
This not only i smy experience, it it is also stated in Apple DateFormatting Doku.