Calculating the number of days between two dates is a trivial matter in T-SQL if you use the DATEDIFF function. However, how many years (or rather, fractions of years) there are between two given dates is a matter of which method (day count convention) you apply. In financial mathematics, a lot of calculations use a 30/360 convention, where you apply certain rules in order to modify each month to contain exactly 30 days.
