I dont seem to be explaining this right but i will try again

I have a user enter a date into an imput box
if the date is valid i need to run it throught a function to determine if it is a leap year or not
i have it worked out so if the valid date is divisible by four it is a leap year if it has decimals its not
i dont know where to take it from there
how can i choose that date is a leap year from
my choices being with or with out decimal
if (vbdate is ??????) then
picbox.print vbdate & " is a leap year."
else
not(vbdate)

picbox.print vbdate & " is not a leap year."
end if