Generally the deadline for contributions is the tax filing deadline, with extensions. In order to deduct your contributions to a Keogh Plan from your taxable income, the Keogh Plan has to be set up by the last day of that year (December 31).

The deadline to make contributions to your Keogh Plan is the same as the due date for Federal Income Taxes for your business. This includes extensions, so you may be able to make contributions until October of the following year.

Interestingly, you may also be able to over-fund a plan in one year and not deduct those contributions until the following years, as long as the cumulative deductions for those years do not exceed limits. Consult with your tax advisor before over-funding a plan, because excess contributions may be subject to a 10% excise penalty tax.

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