Thanks for the reminder. Changed:

on *:text:!attic &:#test:{
if ($2 isnum 0-500) {
%var = $calc(420 + $2)
if (!$timer(attic)) {
.timercountdown 1 $2 msg $chan Text
/timer1 0 %var msg $chan Text
}
}
else { msg $chan Not a number between 0-500 }
}