you need to write one plugin for detect time forma.in IOS by objective c we can detect device time format, same as in java (android) also. so check below code and write one plugin.
IOS
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setLocale:[NSLocale currentLocale]];
[formatter setDateStyle:NSDateFormatterNoStyle];
[formatter setTimeStyle:NSDateFormatterShortStyle];
NSString *dateString = [formatter stringFromDate:[NSDate date]];
NSRange amRange = [dateString rangeOfString:[formatter AMSymbol]];
NSRange pmRange = [dateString rangeOfString:[formatter PMSymbol]];
BOOL is24h = (amRange.location == NSNotFound && pmRange.location == NSNotFound);
NSLog(@"IS 24 h format%@\n",(is24h ? @"YES" : @"NO"));
Android
String value = android.provider.Settings.System.getString(context.getContentResolver(), android.provider.Settings.System.TIME_12_24);