Issues with Datediff

Hi All

Hi All

i am trying to find a difference between dates using a datediff function. But it say type mismatch. can you let me know
what issue in the below code

Dim FD1 As Date
Dim FD2 As Long

FD1= NOW()
FD2 = DateDiff(Format(FD1, "DD/MM/YYYY"), "18 / 3 / 2016", "M")  

MSGBOX FD2

Also is it better to use Dateserial instead of datediff