You need to be on the ULTIMATE plan or the PRO plan with the "Focus timer" feature selected.
The focus timer allows you to track time spent on a task.
For 25 minutes, the timer is in focus mode, where you should be working only on that particular task. Then you will have 5 minutes of rest. This is inspired by the pomodoro technique which will help you focus more while feeling more refreshed thanks to proper resting time.
Click on the task,
Click "Start Timer" button from the task menu button as shown below.
Service d'assistance aux clients par UserEcho