I had an idea to make the function recursive to eliminate some code redundancy...

"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") ?
"Hours between 9/18/2005 13:00 and NOW: " + fnDateDiff("2005/09/18 13:00:00",@DATE+' '+@TIME,"h") ?
"Yesterday's date: " + fnDateDiff(@DATE,-1,"d") ?
"Seconds between midnight and 11:59:59 pm: " + fnDateDiff("00:00:00","23:59:59","s") ?
"Seconds between 11:59:59 pm and midnight: " + fnDateDiff("23:59:59","00:00:00","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,Optional $DateOrInterval,$Interval,$Fix)
Dim $Date1,$Date2,$jDate[2],$jTime[2],$y,$m,$d,$ss,$mm,$hh,$i
$Date1=Split($DateTime," ")
If UBound($Date1)>1 Exit 1901 EndIf
If UBound($Date1)=0
Select
Case InStr($Date1[0],"/") $jDate=Split($Date1[0],"/")
Case InStr($Date1[0],":")
$jDate=Split("1899/12/30","/")
$jTime=Split($Date1[0],":")
Case 1 Exit 1901
EndSelect
Else
If InStr($Date1[0],"/") and InStr($Date1[1],":")
$jDate=Split($Date1[0],"/")
$jTime=Split($Date1[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
$Date1=CDbl($jDate+$jTime)*86400

If VarType($DateOrInterval)
If InStr($DateOrInterval,"/") or InStr($DateOrInterval,":")
$Date2=fnDateDiff($DateOrInterval)
$fnDateDiff=$Date2-$Date1
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

$fnDateDiff=($Date1+$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,'')
$jDate=Iif($y=1899 & $m=12 & $d=30,'',$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
Else
$fnDateDiff=$Date1
EndIf
EndFunction