I tried them but they didnt seem to work. Sorry
Your problem is partially with the timer event. The other problem is the problem statement. Start time is NOT entered on the jobs list form. It is entered by default when the record is created and since that happens on a different form so the suggested changes were all wrong.
You never defined whether the start time could be changed. If it can't be changed, then the field should be permanently locked. If it can be changed, then you need to define under what conditions it can be changed. If the intention is to calculate the elapsed time from when the request is logged until the task is completed, then the start time should never change.
Also, how are you recording the finish of the task? Are there kiosks at every location so as soon as the porter arrives he can click finish? Does each porter have a phone he is using to log the completion? This form would be a nightmare on a phone so that can't be it. It would be awful even on a tablet. If there is some time between recording the finish and the actual finish, you will have to rely on the honesty of the porters.
Just an idea that might help with recording the finish.
1. Create barcodes that the porter can scan at the entrance to each location.
2. Give the porters id's with their personal barcode.
Then the app can reconcile the porter/to location to find the record to mark finished.
This form is poorly implemented
--The tab order is random so you can't tab from control to control
--You have no visual clue what control the focus is in
--There is no validation so records can be saved with missing/invalid data
--Too much scrolling with data entry fields to the right instead of to the left. By the time the user focuses on what he wants to change, the timer event runs and requeries the form making him lose his place.
--This form allows new records to be added and it should NOT since they need to be created in the new job form.
Subbestion:
--Separate the start and end actions into two different forms The first field on the "end" form should be the porter since the porter has only one job at a time. That will be the easiest thing to search for. AND the list should be sorted by porter rather than time or patient. The start action is more difficult. I would add a groupID to group locations to help the porter find the next job nearest to his current location. OR, maybe you could be smart and when he records an end time, offer him the closest jobs to his current location.