Workdays is a new date calculator for the iPhone that should be released for sale on the iTunes store within the next few days. It is essentially an offshoot of Court Days--but this version is geared toward non-attorneys.
Like Court Days, you can calculate a series of "calendar days" to or from a certain date. You can calculate "business days" as well, but unlike Court days, you can chose whether or not to take legal (federal) holidays into consideration. Also, Workdays has a key feature that differentiates it from Court Days: You can set your own "custom holidays" to be evaluated in the date calculations. For example, you can specify that the day before New Years is a holiday.
Workdays is undergoing review by Apple and should be available in a couple of days. It will sell for just $0.99 and should prove to be a useful toll for any business professional. More information and screenshots are available at LawOnMyPhone's website.