I have worked with other project timers that actually paused or stopped if the computer was inactive for a set amount of time (like paused after 10 minutes and then stopped after 30 minutes) and that was my favorite feature. The problem with the Project Timer now is it is easy to forget (making a desktop widget like others have suggested is a good way to keep it in ones visual area) and my team often has to come back in the next morning to stop it and try to change the times. The problem is, if its over a 24 hour period, and you try to change the end time it thinks you are trying to add time over 24 hours, not subtract it so it gives you an error. So everytime someone forgets, we have to delete the time, then manually recreate it wit the correct hours. 

Having this feature will track project time a lot more honestly, people often leave their desk, leaving the timer on. Setting it to pause for inactivity on the computer makes a lot of sense and will fix a major pain point for many users.