Here's a simple timer... though you could probably search the forum and find multiple scripts for countdown timers.

Code:
alias countdown {
  if ($1) {
    .timercountdown 0 3600 DisplayCountDown $1-
  }
  else echo -a ERROR:  Please provide a date for the timer.
}
alias DisplayCountDown {
  if ($ctime($1-) < $ctime) { .timercountdown off | return }
  msg #yourchannel Your event will start in $duration($calc($ctime($1-) - $ctime))
}

Change the "3600" (60 minutes) to whatever time limit you want for the message to display in the channel. Also, change #yourchannel to the channel you want.

USE: /countdown Oct 31 2006 16:00
USE: /countdown Oct 31 2006 4:00pm

You can also use dates like 4/6/2007, but keep in mind that mIRC treats that as European style dates... the month is the 6 and the day is the 4. And you can just include a date or just a time, rather than both if you want to.

Note that if you want months and years, you might consider using the $DateXpander identifier I wrote as it will accurately display months and years... even with leap years. You can find it on mircscripts.org along with information on how to use it for a variety of things.

Finally, if you want to display the information at a time other than just when the timer triggers, just use:

/DisplayCountDown Oct 31 2006 4:00pm

or similar.