Timer / Stopwatch
I am anticipating that some users would like to have a Timer/Stopwatch to start/stop their time for the day.
I am personally not that keen on timers. They are a lot of work to integrate "properly" to allow for multiple start/stop sessions for the day. Otherwise maybe to keep it simple there is only one timer session per day, and it is up the user to "pause" the timer. The margin for error is high if the user is not constantly thinking in terms of starting/stopping/pausing their timer.
My experience with timers and timesheets in the workplace is that everybody does their very best to avoid them. If somebody wants to use ServicePlanner but loves timers, it is very easy to use an external dedicated timer app to drive this.
Anyway I will add it here to see what other users think.