Quick tip if you have an iPhone: Have your phone open so that you can see the "clock" app with all your other apps. You'll notice that it has a second hand that moves. When you finish ordering, screen shot that screen. You'll have the full time at the top of the shot and the second hand, so there's no confusion. When they call your number, screen shot it again. Then do math.
At no point will you look suspicious, you don't have to fumble with discretely starting and stopping a timer, no one will see a stopwatch going on your phone, and you'll have a completely accurate time.