nested DATEDIF funtion in IF function...

Any suggestions on nesting a DATEDIF function in the IF function
A1 has DATE1
B1 has Name
C1 has DATE2

I would like D1 to render:
If B1 has "BOB" then calculate months between A1 and C1

I tried
=IF(B1="BOB",(Datedif(A1,C1,"m"),"") with no success. Thank you for your help.

It worked perfectly.
I cannot stress it enough how much you all have helped me recently. Thank you again for your time and effort.