Date Time Calc (http://www.pomola.com/products_datetimecalc/datetimecalc.html) is a new Mac OS X utility for calculating future and past dates using a given time interval from an assigned date. It requires Mac OS X 10.6.6, costs US$2.99 and is only available at the Mac App Store.
This app is intended for users needing to calculate expected dates after a specific number of days as in a typical business use. An example of periods often used in business: 15, 30, 60, 90, 120 or 150 days.
The dates can be selected using standard calendar controls. The time interval is expressed in days or in a more precise way also using hours, minutes and seconds. A PDf user guide is embedded in the app.