Using a normal timepiece
Beagle, am so pleased you said this. I see more and more instructors/students using stopwatches for visual nav (including one recently who used his mobile phone stopwatch, in airplane mode, - try flying and resetting same with one hand and then where do you put it?!").
A simple analogue timepiece is all that is required and makes additions for eta much easier to calculate.
The same people that use stopwatches when passing "eta" to ATC say "estimating XXX in Y minutes" which is incorrect and sounds unprofessional.