"You spent....Xdays XXhours" for modx manager?

just came into my head it’ll be nice to have a "You spent X days, XX hours and XX minutes at Manager" for stat.
so I can be aware how much time I waste in there moving a box 2px to the left

I saw it here on the SMF header, and also WebloginPE has it for web users.
Is this simply adding a line somewhere and viola or am I looking over my head? did anyone try that maybe and can share?

sorry for the confusion, weblogin has [+user.lastlogin+] not duration.

I found the relevant code in SMF index.template.php file. (around line 253)
SMF has field in it’s db for total_time_logged_in in the user table and I couldn’t find anything like that in modx_user_attributes so I guess I’m giving up