Very configurable countdown till target date/time script for irssi originally ment to countdown to newyears... Among other things, it features multiple time zone support and ability to customize how the countdown is displayed.

Check out the wiki!

Example:
<@eris> !countdown
<@erroris> samy's countdown to legality is ongoing! Currently there are 36 days 06 hours 28 mins 09 seconds till samy is allowed to use interwebs for perrsonal use, and chown all the tubes]

From the header:
# typing !cdwn (or whatever you set cdwn_trigger to) will report
# type !cdwn UTC-5 for EST, !cdwn UTC-8, !cdwn utc+9 for some asian weirdness etc
#
# Settings (default values shown):
# cdwn_target = 2007/1/1 00:00:00 // time/date to count down to USE THIS FORMAT!!!
# cdwn_dly = 1 // time between calls (prevents flooding)
# cdwn_utc = -5 // this computer's timezone in utc format MUST SET THIS!!
# cdwn_trigger = !cdwn // trigger to use in channels
# // These define what to say before and after the target date (can add colors or remove year etc etc)
# cdwn_til_str = $year years $mon months $day days til $tyear/$tmon/$tday $thour:$tmin:$tsec ($utc)
# cdwn_past_str =$year years $mon months $day days past $tyear/$tmon/$tday $thour:$tmin:$tsec ($utc)
#
# Note: $yday differs from $day (which is monthday) by displaying how many calender days
# till the target (as compared to monthdays which will never be > 30)

  Download cdwn.pl.txt, remove the .txt extention and copy to ur .irssi/scripts/ dir (or .irssi/scripts/autorun/ :) and then type /set cdwn to see the settings.

Todo: (please do if u want!! ^_^)
Add channel filter
Add support for multiple countdowns

ENJOY! :D