The simplest way to start and stop a time is to tap on the corresponding project card on the dashboard.

If the project isn’t running just tap the card and it will start running, and stopping the previous (if any) running project.
To stop a project, just tap on the running project card.

If you forgot to start the project at the time you started working on it, you can use the “Start At” feature to start it in the past. To access that function you can use the project cards overflow menu (the three small dots to the right on the card) or you can use the shortcut of a long press on the card to get to the same feature