My wife and I had a party on our mutual 100th birthday—that is, the day that our ages added together was exactly 100 years.
In case you're interested in doing math involving dates, it's really easy if you use Excel or some other spreadsheet. Dates are stored internally as a number representing the number of days since some arbitrary calendar start date—probably January 1 of the year 1. If you subtract one date from another, it gives you the number of days between them. Just be sure your result field is formatted appropriately as either a date or a number field, depending on what you're trying to calculate.