You can add time to a task using the built in timer, or you can manually add time entries
You can access the timer directly in the task list by clicking the clock icon
You can also access the timer from the task details page. When you're in the task details page, click the 'Timer' tab and then click Start Timer.
You can manually enter time on the task details page. Click the 'Timer' tabl and then click 'Enter Time'. This will open a dialog where you can enter time that was tracked outside of Duet.