Here is a script I use to add 25 days to a date.

retVal=oBusObj.GetValue("RequiredExpireDate$",sDate)
retVal = oSession.FormatDate(sDate, newDate, "%M/%D/%Y")
newDate = DateAdd("D", 25, newDate)
newDate = oSession.GetFormattedDate(CStr(newDate))
retVal = oBusObj.SetValue("UDF_HOUSTON_DATE$", newDate)

As you can see I use %M%D%Y  while you use %Y%M%D  change yours around and see if that works.

Thanks for helping BigLouie. As soon as I select an Invoice number (or create a new one) I get an error (before entering a date at all.)

it says "88 Invalid/unknown property name. Wrong number of arguments or invalid property assignment: 'oSession.FormatDate' (err/ret = 2/0)." Here is my current code:

invtime=""
sNowTime=""
cValidFutureDate=""
cValidFuture=""
retVal = oBusObj.GetValue("InvoiceDate$",dInvDate)

cValidFuture = DateAdd("D",5,now)
retVal = oSession.FormatDate(cValidFuture,cValidFutureDate,"%M/%D/%Y")

retVal = oSession.AsObject(oSession.UI).MessageBox(CStr(dInvDate))
retVal = oSession.AsObject(oSession.UI).MessageBox(CStr(cValidFutureDate))

if dInvDate>cValidFutureDate then
retVal = oSession.AsObject(oSession.UI).MessageBox(CStr("Future"))
end if

This is on the AR_InvoiceHeader table on the InvoiceDate field. Post validate script 

I figured it out, thanks!

Well what was it.

Actually I am so close but have one more thing to figure out. My Message of "Future" pops up and works correctly, but when I add the line to set the invoice date back to today's date I get an error. Can you help with this? Thanks.

entryDate=""
checkDate=""
returnDate = ""

retVal = oBusObj.GetValue("InvoiceDate$",entryDate)

retVal = oSession.FormatDate(oSession.SystemDate, returnDate, "%M/%D/%Y")

checkDate = DateAdd("D",30,returnDate)
checkDate = oSession.GetFormattedDate(CStr(checkDate))

if entryDate > checkDate then
retVal = oSession.AsObject(oSession.UI).MessageBox(CStr("Future"))
retVal = oBusObj.SetValue(InvoiceDate, returnDate)
end if

The 2nd last line of your script should be:

retVal = oBusObj.SetValue("InvoiceDate$", returnDate)

I just tried that, the invoice date stays at whatever I switched it to that way as well. The only difference is that this time I did not receive an error.

The SetValue input variable should be in YYYYMMDD format for it to work.