10/15 for calendar year plans. Cut & pasted 1.415(c)-1(b)(6) below:
(B) Date of employer contributions. For purposes of this paragraph (b), employer contributions are not treated as credited to a participant's account for a particular limitation year unless the contributions are actually made to the plan no later than 30 days after the end of the period described in section 404(a)(6) applicable to the taxable year with or within which the particular limitation year ends. If, however, contributions are made by an employer exempt from Federal income tax (including a governmental employer), the contributions must be made to the plan no later than the 15th day of the tenth calendar month following the end of the calendar year or fiscal year (as applicable, depending on the basis on which the employer keeps its books) with or within which the particular limitation year ends. If contributions are made to a plan after the end of the period during which contributions can be made and treated as credited to a participant's account for a particular limitation year, allocations attributable to those contributions are treated as credited to the participant's account for the limitation year during which those contributions are made.