Showing App's Build Date

You might consider using the built-in __DATE__ and __TIME__ macros which will return a string representation of the date and time the app was built. Perhaps they will be of more help to you:

NSString *dateStr = [NSString stringWithUTF8String:__DATE__];
NSString *timeStr = [NSString stringWithUTF8String:__TIME__];

To get build date with format 'yyMMddHHmm' you could try this:

+ (NSString *)GetBuildDate {
    NSString *buildDate;

    // Get build date and time, format to 'yyMMddHHmm'
    NSString *dateStr = [NSString stringWithFormat:@"%@ %@", [NSString stringWithUTF8String:__DATE__], [NSString stringWithUTF8String:__TIME__]];

    // Convert to date
    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
    [dateFormat setDateFormat:@"LLL d yyyy HH:mm:ss"];
    NSDate *date = [dateFormat dateFromString:dateStr];

    // Set output format and convert to string
    [dateFormat setDateFormat:@"yyMMddHHmm"];
    buildDate = [dateFormat stringFromDate:date];

    [dateFormat release];

    return buildDate;
}

Try running the script as a build phase step, rather than a scheme pre-action step, so it's run all the time, regardless of the type of build you are producing.