I think I have most of it figured out now. I did
time ({string field})- time ({DateTime field) to get a numerical value and called it @TimeTest
then I did time (0,0,0)+ ({@TimeTest)) to get the difference
This works if the string time (tech's estimate) is greater than the actual time- i.e the technician finished earlier than estimated.
However, if the actual time field is greater than the tech's estimate (technician finished later thanestimated) it returns a number in the 23:mm time.
For example
Tecnician estiumated completion at 09:12 and actuall finished at 09:05. I get a return of 0:06:11- That is correct.
However, in another example, a different technician estimated a completion time of 09:17 and finished at 09:23:19 and it returns 23:53:41 instead of a difference of about 6 minutes.
ideally, if the tech finished early I would see negative 06:11 and if he went over, positive 6 minutes