DateDiff is a calendar program that allows you to select two different dates, then calculates the number of days, hours, minutes and seconds between them. It can also determines what the date is X number of days before or after a given date.
Determine number of days between two dates
Determine date X days before or after a given date
Date and plus/minus values saved from one use to the next
Fast scroll through months (click, hold and drag on calendar headers)
Week of year displayed in left column on calendars