Keep track of how long you actually spend on something versus how long you had planned to.
Start the timer when you start working on something. When you're done, just check it off.
If something else came up and you forgot to pause the timer, you can edit how long you spent, and re-start the timer if you'd like.
Understanding where your time is actually going will help you make better decisions the next time you're planning how to spend your time.
