There are some exceptions, like for certain grandfathered plans, but otherwise the tax treatment of non-qualified deferred compensation for employees of tax-exempt entities is governed by Section 457.
I've never looked for any regs or guidance on this, but I would treat the plan, for all practical purposes, as having a plan year end as of the last day of February. If leap year, then February 29th. Other years, February 28th. Until someone convinces me otherwise.
I have a hard time imagining that an IRS auditor would give you trouble on this.
BG5150 gave me this tip and I think it is what you want...the oldest of the new posts since I last visited are at the bottom and newest are at the top.