When I did timed games I used a stop watch.
Told team the time limit of the game and that my watch was the final word on game ending time.
When I pushed the start button I announced that the clock is running to both teams, they can now check their watches and write it down.
If a coach ask me how much time was left my reply was you still have time and that I will let you know when you do not.

We're the epitome of local league (only four teams at the moment, all umpires are league players as well) so we're already on a good rapport.

It's not like the coaches don't have watches--they know very well when the game started and how long we play. And as I said I giving regular time checks from about 4th inning (which is usually around 10 minutes left on a 50 minute NNI game).

I basically tell them because we have a mix of rookies who have never played before and very experienced players. At this point in our league we're trying to retain the new players (especially female) and the worst way to do that is having the more experience players use cheap stall tactics to get out of an NNI situation.