Objective-C: Return current date/time in "yyyy-MM-dd HH:mm:ss.SSS Z" format

timestampString.m

// Returns current date/time in "yyyy-MM-dd HH:mm:ss.SSSS Z" format using the
// system timezone.
//
// Note that this includes fractional seconds, unlike the system log, so it
// is useful when precise times are desired in log output.
static NSString* timestampString()
{
    // Re-use one NSDateFormatter instance for all calls.
    // NSDateFormatter is thread-safe for iOS 7+.
    static NSDateFormatter* df;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        df = [[NSDateFormatter alloc] init];
        df.dateFormat = @"yyyy-MM-dd HH:mm:ss.SSSS z";
    });

    return [df stringFromDate:[NSDate date]];
}