Each evening you split the day roughly on a ring, say sixty percent on one client and the rest on another. That takes a few seconds and nobody needs a stopwatch.
At the end of the month Stundas turns those rough splits into timesheet bookings in clean half-hour steps. Billable projects land on whole multiples of four hours, the non-billable project absorbs the rounding, and every day still adds up to what you actually worked.