From the time you have them report till the time they get back to the shop and leave.
Now if you have them start at 8 and they arrive at 7:30 the time spent sitting and drinking coffee they are off the clock.
They start at 8. You have them in the truck 8:01. They are on the clock even if the drive time to the first job is 2 hours.
They start at 8. You let them sit around and drink coffee till they get in the truck at 10. They have been on the clock since 8.
Last job ended 4:30, :30 to get back to shop. On the clock till 5.
Back at the shop at 5. If an 8 hour day/40 hour week. You have them clean equipment, gas, whatever anything past 5 then it must be over time.