Create timer using millis()

hai i have timer program but it counts HH:MM:SS but i want seconds with decimal digits like 10.0 09.9 09.8 09.7 as it counts please help me

i have timer program

...but I decided not to show it to you.

Just count tenths of a second instead of seconds. Then use .print(TenthsOfASecond / 10.0, 1) to display with one decimal place.