datetime - How can I determine if iPhone is set for 12 hour or 24 hour time display?

Question

I thought I saw something answering this on SO recently but now I can't find it. Here is the code I am using now to determine if settings are for 24 hour time display. It works for me in the US, but I don't know if it will work in all locales. Is this sufficient or is there a better way to find out the current setting for this?

+(BOOL) use24HourClock
{
    BOOL using24HourClock = NO;

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];    
    [dateFormatter setLocale: [NSLocale currentLocale]];    
    [dateFormatter setDateStyle:kCFDateFormatterNoStyle];
    [dateFormatter setTimeStyle:kCFDateFormatterShortStyle];    
    // get date/time (1Jan2001 0000UTC)
    NSDate* midnight = [[NSDate alloc] initWithTimeIntervalSinceReferenceDate:0];   
    NSString* dateString = [dateFormatter stringFromDate: midnight];
    // dateString will either be "15:00" or "16:00" (depending on DST) or
    // it will be "4:00 PM" or "3:00 PM" (depending on DST)
    using24HourClock = ([dateString length] == 5);
    [midnight release];
    [dateFormatter release];    

    return using24HourClock;
}

Answer 1

Here's the best way to do it:

NSString *formatStringForHours = [NSDateFormatter dateFormatFromTemplate:@"j" options:0 locale:[NSLocale currentLocale]];

NSRange containsA = [formatStringForHours rangeOfString:@"a"];
BOOL hasAMPM = containsA.location != NSNotFound;

in Swift:

let formatString: NSString = NSDateFormatter.dateFormatFromTemplate("j", options: 0, locale: NSLocale.currentLocale())!
let hasAMPM = formatString.containsString("a")

Swift 4:

let formatString = DateFormatter.dateFormat(fromTemplate: "j", options: 0, locale: Locale.current)!
let hasAMPM = formatString.contains("a")

This uses a special date template string called "j". According to the ICU Spec, "j"...

requests the preferred hour format for the locale (h, H, K, or k), as determined by whether h, H, K, or k is used in the standard short time format for the locale. In the implementation of such an API, 'j' must be replaced by h, H, K, or k before beginning a match against availableFormats data. Note that use of 'j' in a skeleton passed to an API is the only way to have a skeleton request a locale's preferred time cycle type (12-hour or 24-hour).

That last sentence is important. It "is the only way to have a skeleton request a locale's preferred time cycle type". Since NSDateFormatter and NSCalendar are built on the ICU library, the same holds true here.