OK, one of my moderators saw this on another forum, but I checked and they were running 3.0.4.
In the postbit_legacy was a field for "Last seen:" and had the same information as in the profile "Last Activity:" . We've had so many people drag up old, old threads, that we agreed that it'd be a great idea!
So, I've got the text there, but the variable [lastactivitydate] is blank.
I've inserted into postbit_legacy: <if condition="$post['lastactivity']"><div>$vbphrase[last_seen] $post[lastactivitydate]</div></if>
where last_seen is "Last Seen".
It's at
http://www.curvesforum.com if anyone has ideas!
ETA (since it merged itself):
Never mind, I found it:
https://vborg.vbsupport.ru/showthread.php?t=82623
Thanks for all the help!