They largely didn't beyond general times like midday, sun-up, etc. Britain didn't have a standardized time (they used local mean time which varied by place) until 1847 (railroads were the impetus to developing standard time schemes).

Waterloo: Myth and Reality. Gareth Glover. P. 110

“When the first cannon roared at the beginning of the Battle of Waterloo, every man on the field with a pocket watch took note of the time, and everyone else just guessed. Unfortunately no one thought to synchronise watches and the times quoted for the opening salvo vary from 9 a.m. to noon. However, it is as certain as it can be that the battle commenced at approximately 11:30 a.m.”

In the Austerlitz campaign, the Austrians and Russians couldn't even coordinate what day it was because they used different calendars!