This pulls the unix timestamp from the U_Laston field of the user. This field is set and updated by .threads itself while users use the forum. I believe the only script that updates this time is the addpost.php script and ubbt.inc.php when a user logs in.
I know it's not accurate and this is compounded by the fact that it gets cached. A new field would be needed for the users table and one extra query would be needed during the send_header call to update this field with a timestamp to provide accurate "last seen" information. I don't know if such a modification would be wanted by all but I'll add the directions for the next release as an option for those that do.
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