Restrict Data In Input Box

How can I restrict the user from entering anything other than the date and the proper format required for this input box? I am having a tough time figuring out how to check the input for the right date and format . .

Sub EffDate()
Dim a As Variant

a = InputBox("Enter the effective date" & _
" of the subject's lease using a 2 digit day," & _
" a 2 digit month and a 4 digit year" & _
" (dd/mm/yyyy).", "C/NC Analysis")

Range("U4").Value = a
End Sub