The purpose is to start it at the start of a workday to improve productivity. So the timer should also sleep when the PC is sleeping. It would be good if the app displays the remaining time in the dock or menu bar.
macOS has a built-in program that keeps track of your time. This program calculates the average amount of time you spent at work. And every week, it sends you a report where you can also see the daily dynamics and which applications you have used the most. But maybe I misunderstood your question.