The main interface of this application is Cocoa and it appears to be written in Objective-C. In one window it provides a calendar-and-clock style "date picker" interface object, similar to what's used in the Date & Time preference pane of System Preferences. This interface object is also known as an "NSDatePicker" object, and Apple first introduced it in OS X 10.4. So, in OS X 10.4 it's extremely easy for a developer to show a calendar, a simple drag and drop in Interface Builder and a few lines of code, and you're set. Prior to OS X 10.4, a developer could spend months trying to create a similar interface themselves. Versions of OS X 10.3 and earlier are unable display these types of objects which are created in later versions of OS X.