The various deadlines for retirement plans, IRAs and Coverdells are noted below.
Generally, employer plans must be established (applications executed) by Dec. 31 to get a 2011 deduction. This includes Keoghs (these still exist?).
For SEPs, you have until the due date for filing your return plus any extension. Thus, self-employeds who miss the Keogh setup deadline will have more time to open (or miss this deadline) SEP-IRAs. Contributions for 2011 are due by Oct. 17, 2012. Keoghs and SEPs have the same payin cap: 20% of net self-employment earnings. The net profit shown on your Schedule C, less one-half of your (SECA) Self Employment Contribution Act tax liability.
Regular IRAs must be established by April 16, 2012 for 2011 deductions. Payins are due by then as well. A filing extension will not buy you additional time.
Nondeductible payins to IRAs and Roth IRAs are also due by April 16. Ditto for contributions made to Coverdell education savings accounts.