I have a handful of clients that don't know the meaning of April 15th and I won't see them or any of their information until several months after the due date.  They assume I will take care of an extension for them --------- and I do, even though I despise babysitting clients.  For those folks I will use the prior year return to estimate where they will come out ---------- which is normally pretty close to the final numbers when the return is ultimately filed.