while it was suggested that /.timer 0 20 was better because mIRC would assign its own number and this was better than using named timers, i disagree
However it may be better to use a name that is more distictive to the alias/script
in this case auto greeting
on me:*:join:#pokenightmare: .timerautomsg 0 20 .notice Hello and welcome to #pokenightmare.
It is important to note the . is used here so that your bot's log isnt filled with pages of the notice going out.
for testing use
on me:*:join:#pokenightmare: timerautomsg 0 20 notice Hello and welcome to #pokenightmare.
frankly i think 20 seconds is a bit often maybe 300 would be better