The issue is not whether the target date is inside/outside summertime, but whether it's in the opposite condition as the current time.

Code:
alias datefix {
  var -s %a $replace(2013-01-10T13:13:13Z,t,$chr(32),z,$chr(32))
  var -s %b $date $gettok(%a,2,32)
  var -s %a2 $ctime(%a) , %b2 $ctime(%b)
  var -s %fix $calc( (%b2 - %a2) % 86400)
  return %fix
}


This will return
82800 = today is during summertime, target day is not
0 = today and target day are either both inside or both outside summertime
3600 = target day is within summertime, today is not

And as I mentioned in my post, if either today or the target day, or both, is one of the twice per year day of transition between on/off summertime, it might need further adjustments to the formula.