http://www.complianceheadquarters.com/IRA/IRA_Articles/simplified_employee_pensions.html

Therefore, most employers make SEP contributions after the end of the calendar year, the model SEP's required plan year. This coincides with many employers' tax years. SEP rules allow employer contributions made by the due date of an employer's tax return (plus extensions) to be considered made at the end of the calendar plan year.

The tax return due date is April 15 for unincorporated calendar year businesses, with extensions available to August 15 automatically and as late as October 15 with IRS approval. For calendar-year corporations and partnerships, this due date is March 15, with an automatic extension available to September 15.