An electronic stop watch that is thumb operated is a poor way to measure accurate times. You need a suitable light, good fast photocell, amplifier, then a Schmidt trigger or comparator to gate a counter of standard frequency quartz crystal oscillations (3rd rate here, use WWV for higher acuracy). Just use a decimal point after every two digits. hours.min.sec.sec/100.sec/10000.sec/1000000, etc.

Of course if the first is a millenium or so long you have to use more than two digits for the hours part. Just remember that all before the 1st decimal point is hours.