|FirstDate||First date to find the time between.|
|SecondDate||Second date time find the time between.|
Note that the return value will always be non-negative, no matter whether the first or second date comes first.
Note that while this returns an absolute value result, the caller can also use
DateIsAfter to determine whether the first date is before or after the second. May want to do this to differentiate between when a date has already gone past versus when it is still to come.