How would one go about measuring session length?

[mabdullahabid]

There seems to be multiple ways to do it so I'd like to know what everyone thinks.

[dwasyl]

I'd added that functionality by adding a created_at field to the Session model (pushed as PR #75 but I couldn't get it to pass the tests due to some DB issues. It was the only way to measure session length I could come up with, and useful for being able to tell when a session was first created. What other ways would you do this? I haven't had time to rewrite it without the auto_add_now field since I've just been using my older fork.