I don't think there's a function, but you could do it like this....
1) I take it that a session get registered when somebody loggs in. Soo....
When the session get's registerd, grab the Unix TimeStamp using the time(); function. Using that, you can format it to tell you when they logged in.
On my site I have something like that. More like an online system and that's what I did. Give it a try, and see if it helps you in anyway.
Stereo
