I use my digital watch (which I wear only on shops) and the timer on my phone. Since my phone's lap timer has a propensity to restart itself, stop timing if I accidentally bump my handbag on something, or lose the time before I can save it, the watch is a great double check.
I was paranoid about the timing, too, when I did my first one, but you'll be fine! The shops overall get much easier the more of them that you do.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.