|FirstDatetime||The first of the two datetimes to find the amount of time between.|
|SecondDatetime||The second of the two datetimes to find the amount of time between.|
Note that the return value will always be non-negative, no matter whether the first or second datetime comes first.
Note that while this does return an absolute value result, the caller can also use
DatetimeIsAfter to determine whether the first datetime is before or after the second. May want to do this to differentiate between when time has already gone past versus when it is still to come.