iOS APp to o calculate number of days between two dates

In addition to NSDate to find the time interval between two dates, you'll also want to use an NSDatePicker so that the user can input two dates:

http://developer.apple.com/library/ios/documentation/uikit/reference/UIDatePicker_Class/

Finally, some basic knowledge about how integer division and modulus work so that you can convert the time interval to seconds, minutes, hours, and days will help... Unless UIKit happens to have a method that does that already built in? I'm not sure which class would hold such a method...

Don't forget to add a little button that includes or excludes the final day.

This is very suspect of a class assignment.