I am still not sure if I understand as you need a start time and and end time to get an elapsed time. I am going to assume that Scan Date is when a Scan started and Data Date is when the Scan finished. If that is the case, what you want to do is get the difference between the two, sum up this difference and then divide by your number. The final result will be in seconds but you should be able to get hours, minutes and seconds out of that.

This will return the number of Seconds between the start time of ScanDatetime and DateDatetime:

DATEDIFF(S,ScanDatetime, DataDatetime)

From there you should be able to add, subtract, average or do anything else you like with it.

If this is not what you want, and ScanDatetime and DateDatetime are not the starting and stopping times, you still need something to determine elapsed time.