On some of the DVRs you can adjust the timer to count up or count down. Check the manual. Before I got one that could upload to my computer to easily jump around without fast forward or rewind on the actual DVR, I used to make a series of coughs, clicks etc. to mark the beginning of timings. At home when I listened, I could use my stopwatch to measure elapsed time between the sound marks.
So you are timing from the time you join the line until the time your food is delivered with an intermediate timing of when they total your order.
Cough to start when you join the line, you can hear when they total your order and note it from your stop watch while the watch then ticks on through to your "thanks" when you are presented with your order.
As for button control, they are all somewhat different. Mine has a square button on the left of the front that starts/pauses recording and a square button on the right of the front that ends the recording. I make sure it is turned on in the parking lot, push the start as I approach the front door and do a quick visual to make sure the record light is on and not blinking, and tell it the time in. It is easy enough to start earlier and comment about external maintenance issues and stores to the left and right if needed. If it is a short shop I will leave it turned on until I leave and as I am heading for the car, tell it what time I am out, name/description of cashier and anyone else I need. I work on the assumption that greetings and parting remarks are on the recording already.