IPhone : How to compare system date with UIDatePicker's date in iOS?

Through my App users can order specific food item in restaurants..The thing is that if a user want to order a specific product he can give atleast 6 hrs time for the restaurant to deliver the product.For example you are the user,right now time is 12:00pm..In the app,user can select the timings of DELIVERY through UIDatepicker.Here if user selects the time before 06:00pm,one alert view will generate with message "please give 6hrs time for your order".Here i have to compare system time and UIDatepickers time.I found one method,,but it is not useful.

if ([self.datePicker.date compare:currentdate] == NSOrderedDescending){
   NSLog(@"Descending");
}

Please help me...Thanks in advance