How can I simply and reliably track my computer usage?
I've used RescueTime for years and it's been an informative little app. 

They present your time consumption on a nice dashboard and you can categorize it and rate the productivity of your activities from "Very distractive" to "Very productive", set goals, etc.



If you wish they can send you a weekly digest email.

From what I've read on their forums –I don't watch movies on my computer– YMMV with movies; VLC seems to have problems being captured after the defined idle time, which is 5 minutes and other similar activities may have the same problem. Hangouts appeared as problematic while searching for "movies" on their site to answer this. 

This is what they say regarding idle time:


  When RescueTime detects that your computer is idle (no keyboard or mouse activity), it begins a 5 minute timer. At the end of five full minutes of inactivity, RescueTime will close the most recent record and subtract 5 minutes of time from that record.


If you are willing to pay, they offer a premium subscription that allows you to edit your idle time:


  If you are a Premium user and have offline time enabled, you will get a pop up after becoming active again that will allow you to enter what you were doing when away from the computer, unless this time exceeds 4 hours. If your time away is greater than 4 hours, you can enter time manually on the Offline Time Entry page


It works on OS X and Windows.