How do you get the time interval for UIDatePicker in UIDatePickerModeCountDownTimer mode?

I have a UIViewController with a UIDatePicker datePicker property. I have configured it in Interface Builder to have a UIDatePicker in "Timer" mode and to have it call the datePickerValueDidChange: method of the view c开发者_运维问答ontroller when the "Value Changed" event occurs. The code in the view controller looks like this:

- (void) viewWillAppear:(BOOL)animated {
  NSLog(@"datePicker: %@", datePicker);
  NSLog(@"datePicker.datePickerMode: %d", datePicker.datePickerMode);
}

And when the view loads, you see this in the log

datePicker: <UIDatePicker: 0x3b03660; frame = (0 61; 320 216); clipsToBounds = YES; opaque = NO; autoresize = RM+BM; layer = <CALayer: 0x3b033a0>>
datePicker.datePickerMode: 3

So that means the date picker is initialized and the mode is UIDatePickerModeCountDownTimer. When I change the value by moving one of the scroll wheels, it calls this method:

- (IBAction) datePickerValueDidChange:(id)sender {
  NSLog(@"datePicker.countDownDuration: %@", datePicker.countDownDuration);
}

And the output in the log is:

datePicker.countDownDuration: (null)

Why is it null? Is this not the way to get the value for the hours/minutes the user has selected? If not, how can you get those values?

countDownDuration is an NSTimeInterval (really a double). To print it out, you'll need to use

NSLog(@"datePicker.countDownDuration: %f", datePicker.countDownDuration);