I wish to ammend my suggestion...
.opened : returns the $ctime value when $chan $query $chat were opened.
.joined : returns the $ctime value when $nick joined $chan. ($nick($chan,$nick).joined)
Instead, I wish for these to return the number of seconds since they joined or window creation... identical to .idle.
Example:
$nick(Raccoon,#mIRC).joined == 5
Raccoon joined 5 seconds ago.
If the user was in the channel before you joined, it should return $null