Yes you can!
You can start and log the timer as many times as you like on a task. This may happen when you do sporadic work on a task. The "total time logged" box will show you the accumulated amount of time.
Always remember to NOT navigate away from the page while the time is running (if you need to open another web page, then do so in another window or tab) or you will lose your time.
For more information about your task timer: