Date Difference just gives the difference between two dates and/or times, and it tells you the length of time between them. There are preferences for what units to enable or disable (days, hours, minutes, and/or seconds), and it compensates for the disabled units with the next largest enabled unit (for example, if the difference between the dates is 1 day and 2 hours, but you disable days, then it becomes 26 hours).
Fixed problems with compatibility in OS X 10.4 (i.e., not launching)
Fixed problems with time intervals being too long for the computer to keep track of (dates before sometime around 1940 returned a negative result)
Added option to use zeroes or not (meaning if there are 0 days in between the two dates, by default Date Difference no longer displays them) This can be changed in the preferences.