DateDiff problem

In Authorware there is the function "DateToNum" here are the specifications:

number := DateToNum(day; month; year)
DateToNum converts a date (day, month, and year) to a number that corresponds to the number of days since January 1, 1900. This function performs the opposite calculation from the Date function, which converts a number to a date. The following table shows the range of valid dates:
Windows: 1, 1, 1970, to June, 2, 2036 (25568 to 49709)
Macintosh: 1, 1, 1904, to June, 2, 2040 (1461 to 51171)

I tried to code it in vb (asp actually) but it is not the same :

function DateToNum(dtDate)
dim strStartDate, dtStartDate
strStartDate = "January 1, 1900"
dtStartDate = DateValue(strStartDate)
DateToNum = DateDiff("d",dtStartDate,dtDate)
end function

1/1/1970 produces 25568 in authorware but 25567 in my routine.

How can I make the routine work for me ?

DateToNum = DateDiff("d",dtStartDate+1,dtDate)

try this:

VB Code:

Private Sub Command1_Click()
    MsgBox DateToNum("01/01/1970")
End Sub
 
Public Function DateToNum(pDate As Date) As Long
    Dim lngDate As Long
    
    lngDate = pDate
    
    DateToNum = lngDate
End Function

Both solutions do not produce equivalent output. I converted nullus's routine to asp:

function DateToNum(dtDate)
Dim lngDate
lngDate = CLng(CDate(dtDate))
DateToNum = lngDate
end function

however this gives me 25569 for 1/1/1970, while the authorware routine returns 25568.
inserting "June 2, 2036" returns 49828 while in authorware it returns 49709.

It works... Authorware goes beserk out of the bounds of 1970, 2036.. Here's my code :

function DateToNum(dtDate)
Dim lngDate
lngDate = CLng(CDate(dtDate))
DateToNum = lngDate-1
end function

function NumToDate(lngNumDate)
dim dtDate
dtDate = Cdate(CLng(lngNumDate))
NumToDate = dtDate+1
end function

Response.Write DateToNum("September 5, 2002")
Response.Write "<br />"
Response.Write NumToDate(37503)