2010 New York Code
GOB - General Obligations
Article 5 - CREATION, DEFINITION AND ENFORCEMENT OF CONTRACTUAL OBLIGATIONS
Title 11 - (5-1101 - 5-1115) CONSIDERATION; EFFECT OF WRITING ON REQUIREMENT OF CONSIDERATION
5-1109 - Written irrevocable offer.

§ 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.