Get a standard sports watch, £10 or so from argos. Either one with an easy start stop, or possibly big numbers, so you can see the time at a glance if you don't use stop /start (like me, as I'm always forgetting to stop /start the watch).
Then add on the added time at the end in your head going by subs, retrieval of ball from bushes etc.
Obviously, this is grass roots level I'm on about.
*Edit - if you are doing it without stop start, always note down time of first and second half ko, either in book or on back of hand.
I was a bugger for not doing that in my first couple of games.