Lonk, you're a prick, but here is a version that doesn't require anything but KiX.

"One second after midnight: " + fnDateDiff(@DATE,1,"s") ?
"One second before midnight: " + fnDateDiff(@DATE,-1,"s") ?
"Seconds between today and tomorrow: " + fnDateDiff(@DATE,fnDateDiff(@DATE,1,"d"),"s",0) ?
"Weeks between today and six days from today: " + fnDateDiff(@DATE,fnDateDiff(@DATE,6,"d"),"w",0) ?
"Weeks (fixed) between today and six days from today: " + fnDateDiff(@DATE,fnDateDiff(@DATE,6,"d"),"w",1) ?
"Shopping days until my birthday: " + fnDateDiff(@DATE,"2006/04/20","d") ?
"Seconds between 1/1/1601 and 1/1/1970: " + fnDateDiff("1601/01/01","1970/01/01","s") ?
"Hours between NOW and 9/18/2005 13:00: " + fnDateDiff(@DATE+' '+@TIME,"2005/09/18 13:00:00","h") ?
"Yesterday's date: " + fnDateDiff(@DATE,-1,"d") ?
"Seconds between midnight and 11:59:59 pm: " + fnDateDiff("00:00:00","23:59:59","s") ?
"Add one minute to midnight: " + fnDateDiff("00:00:00",1,"m",0) ?
"VarTypeName of fnDateDiff output: " + VarTypeName(fnDateDiff(@TIME,0,"s")) ?
"VarTypeName of @@TIME: " + VarTypeName(@TIME) ?
@SERROR ?
 
Get $
 
Function fnDateDiff($DateTime,$DateOrInterval,Optional $Interval,$Fix)
   Dim $jDate[2],$jTime[2],$y,$m,$d,$ss,$mm,$hh,$1899,$i
   $DateTime=Split($DateTime," ")
   If UBound($DateTime)>1 Exit 1901 EndIf
   If UBound($DateTime)=0
      Select
      Case InStr($DateTime[0],"/") $jDate=Split($DateTime[0],"/")
      Case InStr($DateTime[0],":") $jDate=Split("1899/12/30","/") $jTime=Split($DateTime[0],":") $1899=1
      Case 1 Exit 1901
      EndSelect
   Else
      If InStr($DateTime[0],"/") and InStr($DateTime[1],":")
         $jDate=Split($DateTime[0],"/") 
         $jTime=Split($DateTime[1],":")
      Else
         Exit 1901
      EndIf
   EndIf
   $y=Val($jDate[0]) If $y=0 Exit 1901 EndIf
   $m=Val($jDate[1]) If $m<1 or $m>12 Exit 1901 EndIf
   $d=Val($jDate[2]) If $d<1 or $d>31 Exit 1901 EndIf
   If $m < 3 $m=$m+12 $y=$y-1 EndIf
   $jDate = CDbl($d+(153*$m-457)/5+365*$y+$y/4-$y/100+$y/400-306)
   $jTime = CDbl((Val($jTime[0])*3600)+(Val($jTime[1])*60)+Val($jTime[2]))/86400
   If $jTime=>1 Exit 1901 EndIf
   $DateTime=CDbl($jDate+$jTime)*86400
   
   If InStr($DateOrInterval,"/") or InStr($DateOrInterval,":")
      ReDim $jDate[2],$jTime[2],$y,$m,$d
      $DateOrInterval=Split($DateOrInterval," ")
      If UBound($DateOrInterval)>1 Exit 1901 EndIf
      If UBound($DateOrInterval)=0
         Select
         Case InStr($DateOrInterval[0],"/") $jDate=Split($DateOrInterval[0],"/")
         Case InStr($DateOrInterval[0],":")
            $jTime=Split($DateOrInterval[0],":")
            $jDate=Split("1899/12/30","/")
            If $1899=0 Exit 1804 EndIf
         Case 1 Exit 1901
         EndSelect
      Else
         If InStr($DateOrInterval[0],"/") and InStr($DateOrInterval[1],":")
            $jDate=Split($DateOrInterval[0],"/") 
            $jTime=Split($DateOrInterval[1],":")
         Else
            Exit 1901
         EndIf
      EndIf
      $y=Val($jDate[0]) If $y=0 Exit 1901 EndIf
      $m=Val($jDate[1]) If $m<1 or $m>12 Exit 1901 EndIf
      $d=Val($jDate[2]) If $d<1 or $d>31 Exit 1901 EndIf
      If $m < 3 $m=$m+12 $y=$y-1 EndIf
      $jDate = CDbl($d+(153*$m-457)/5+365*$y+$y/4-$y/100+$y/400-306)
      $jTime = CDbl((Val($jTime[0])*3600)+(Val($jTime[1])*60)+Val($jTime[2]))/86400
      If $jTime>1 Exit 1901 EndIf
      $DateOrInterval=CDbl($jDate+$jTime)*86400
      $fnDateDiff=$DateOrInterval-$DateTime
      Select
      Case $Interval="s"
      Case $Interval="m" $fnDateDiff=$fnDateDiff/60
      Case $Interval="h" $fnDateDiff=$fnDateDiff/3600
      Case $Interval="d" $fnDateDiff=$fnDateDiff/86400
      Case $Interval="w" $fnDateDiff=$fnDateDiff/604800
      Case 1 Exit 87
      EndSelect
      If $Interval="s" $fnDateDiff=CInt($fnDateDiff) EndIf
      If $Fix $fnDateDiff=Fix($fnDateDiff) EndIf
   Else
      If VarType($DateOrInterval)>5 Exit 87 EndIf
      Select
      Case $Interval="s" If VarType($DateOrInterval)>3 Exit 87 EndIf
      Case $Interval="m" $DateOrInterval=$DateOrInterval*60
      Case $Interval="h" $DateOrInterval=$DateOrInterval*3600
      Case $Interval="d" $DateOrInterval=$DateOrInterval*86400
      Case $Interval="w" $DateOrInterval=$DateOrInterval*604800
      Case 1 Exit 87
      EndSelect
      
      $fnDateDiff=($DateTime+$DateOrInterval)/86400
      $jTime=$fnDateDiff-Fix($fnDateDiff)
      $jDate=Fix($fnDateDiff)
      
      $y = (100*(((100*($jDate+306)-25)/3652425)-(((100*($jDate+306)-25)/3652425)/4))+
         (100*($jDate+306)-25))/36525 
      $m = (5*(((100*($jDate+306)-25)/3652425)-(((100*($jDate+306)-25)/3652425)/4)+
         ($jDate+306)-365*$y-$y/4)+456)/153 
      $d = (((100*($jDate+306)-25)/3652425)-(((100*($jDate+306)-25)/3652425)/4)+
         ($jDate+306)-365*$y-$y/4)-(153*$m-457)/5 
      If $m>12 $y=$y+1 $m=$m-12 EndIf
      If Len($y)<4 $ = Execute("For $$i=1 to 4-len($$y) $$y = '0' + $$y Next") EndIf
      $m = Right("0"+$m,2) 
      $d = Right("0"+$d,2)
      
      $ss = CInt(86400.0*$jTime)
      $mm = $ss / 60
      $ss = $ss mod 60
      $hh = $mm / 60
      $mm = $mm mod 60
      
      $jDate=Iif($y,''+$y+'/'+$m+'/'+$d,'')
      $jDate=Iif($1899,'',$jDate)
      $jTime=Iif($jTime,Right("0"+$hh,2)+':'+Right("0"+$mm,2)+':'+Right("0"+$ss,2),'')
      
      $fnDateDiff=Iif($jDate & $jTime,$jDate+" "+$jTime,$jDate+$jTime)
   EndIf
EndFunction