I have tried DateXpander. Which can be found here: https://forums.mirc.com/ubbthreads....uration-include-months-years#Post126269.
Changed the code to calculate out 31, which is the end of the year here in my Country.
But always it return 1 day till end of the year.

So I tried to code this from the ground. The snippet below should return 2 days, but in mIRC it returns 1 day.
Code
alias EndOfYear {
  ; Calculate the end of the year time
  var %end_of_year $ctime($asctime(yyyy) $+ /12/31 00:00)

  echo -ag $duration($calc(%end_of_year - $ctime))
}

I'm totally lost, maybe I code wrong?
I have not used MSL since a long time ago.