Check if date is DST with AMPscript

isdst.amp

%%[

  set @dateToCheck = "2023-10-25"

  set @month = datepart(@dateToCheck, "M")
  set @day = datepart(@dateToCheck, "D")
  set @daysOfWeek = "SunMonTueWedThuFriSat"
  set @dayOfWeekToFind = format(@dateToCheck,"ddd")
  set @dayOfWeekNum = divide(add(indexof(@daysOfWeek,@dayOfWeekToFind),2),3)

  if @month < 3 or @month > 11 then
    set @isDST = 0
  elseif @month > 3 and @month < 10 then
    set @isDST = 1
  else
      set @prevSunday = subtract(@day, subtract(@dayOfWeekNum,1))
      if @month == 3 then
        if @prevSunday >= 8 then
          set @isDST = 1
        else
          set @isDST = 0
        endif
      else
         if @prevSunday <= 0 then
            set @isDST = 1
         else
            set @isDST = 0
         endif
      endif
  endif

  output(concat("<hr>dateToCheck: ", @dateToCheck))
  output(concat("<br>month: ", @month))
  output(concat("<br>day: ", @day))
  output(concat("<br>dayOfWeekToFind: ", @dayOfWeekToFind))
  output(concat("<br>dayOfWeekNum: ", @dayOfWeekNum))
  output(concat("<br>prevSunday: ", @prevSunday))
  output(concat("<br>isDST: ", @isDST))

]%%