Not crazy. The deadline for the employer contribution is the tax due date for the business entity (not necessarily 4/15). To be eligible to make these contributions after the due date, the taxpayer must have filed an extension, and an extension cannot be filed after the return has been filed. Therefore, if the return is filed on time (or late without an extension), the taxpayer is not allowed to make a contribution after the initial due date. Finally, since the tax due can vary for certain non sole proprietor business entities, a 6 month extension could end prior to 10/15 (9/15 if the business entity had a due date of 3/15). This also applies to a SEP IRA contribution where all contributions are employer contributions.