From -
http://www.irs.gov/pub/irs-pdf/p4334.pdfDepositing and Investing Plan Contributions Employee contributions must be deposited in the financial institution serving as trustee for the plan within 30 days after the end of the month in which the amounts would other wise have been payable to the employee in cash. Your employer contributions must be made by the due date (including extensions) for filing your business’s Federal income tax return for the year.
