Code:
on *:join:#: {
if (!$readini(joins.ini,#,$nick)) writeini joins.ini # $nick $ctime
}
on *:text:!joined:#: {
var %date $readini(joins.ini,#,$nick) 
if (!%date) var %msg Error: No data.
else var %msg You joined $asctime(%date) $+ , $duration(%date) ago.
msg # %msg
}
Be careful however, these kinds of scripts will fill an ini file quite quick and will turn extremely slow towards the end. If you want it to show how long a user has been in the channel lately, since last join that is, you just remove the IF check on the join event and leave it with only writeini.

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Nillen @ irc.rizon.net