time.time()
Return the time in seconds since the epoch as a floating point number. Note that even though the time is always returned as a floating point number, not all systems provide time with a better precision than 1 second. While this function normally returns non-decreasing values, it can return a lower value than a previous call if the system clock has been set back between the two calls.

На кнопке старт делаете: start_time = time.time()
На кнопке финиш ставите: end_time = time.time()
А затем на финише вычитаете конечное время минус время старта:
end_time - start_time