N.Y. Comp. Codes R. & Regs. tit. 9, § 4051.4 – Closing time for entries.

(a) The list of entries shall be closed at the advertised time, and no entry shall be admitted after that time, unless (in other than an overnight race) the nominator can prove within a reasonable time to the satisfaction of the stewards or the stewards of the National Steeplechase and Hunt Association that the entry was mailed or telegraphed before the advertised time of closing.
(b) In case of an emergency the racing secretary may grant an extension of time for closing entries, with the consent of a steward.