What is the deadline for a self-employed individual or partner to make his or her elective deferral contribution?
Rev. 05/01/03, E-mail Alert 2003-8

The compensation for a self-employed individual is not treated as “currently available” until the end of the sole proprietor’s or partnership’s tax year. A salary deferral election form indicating the percentage or dollar amount of the individual’s elective deferral may be completed up until the last day of the plan year. If the deferral election is made before the end of the plan year, the self-employed individual generally has until his or her tax filing deadline, plus extensions, to make the contribution.

2005 Final 401(k) Regulations