New York Consolidated Laws, General Obligations Law - GOB § 5-1109. Written irrevocable offer

Except as otherwise provided in section 2-205 of the uniform commercial code with respect to an offer by a merchant to buy or sell goods, when an offer to enter into a contract is made in a writing signed by the offeror, or by his agent, which states that the offer is irrevocable during a period set forth or until a time fixed, the offer shall not be revocable during such period or until such time because of the absence of consideration for the assurance of irrevocability.  When such a writing states that the offer is irrevocable but does not state any period or time of irrevocability, it shall be construed to state that the offer is irrevocable for a reasonable time.