Ended up with this which works [Eek!]

;----- This compares the virus def date
;----- And the definitions date
;----- Returns the days between the two dates

Function JulianDate($TxDate) ; Assumes 4 digit year, M/D/Y or Y/M/D and / as delimiter
DIM $Year, $Month, $Day, $Pos
$Pos=INSTR($TxDate,"/")
If $Pos=5
$Year=0+SUBSTR($TxDate,1,4)
Else
$Month=0+SUBSTR($TxDate,1,$Pos-1)
EndIf
$TxDate=SUBSTR($TxDate,1+$Pos,10)
$Pos=INSTR($TxDate,"/")
If $Year
$Month=0+SUBSTR($TxDate,1,$Pos-1)
$Day=0+SUBSTR($TxDate,1+$Pos,2)
Else
$Day=0+SUBSTR($TxDate,1,$Pos-1)
$Year=0+SUBSTR($TxDate,1+$Pos,4)
EndIf
$JulianDate=367*$Year-7*($Year+(($Month+9)/12))/4-3*((($Year+($Month-9)/7)/100)+1)/4+275*$Month/9+$Day-678972
EndFunction

$daysbetween=JulianDate(@date)-JulianDate($defdates)