N.Y. Police Certain Municipalities Law § 2
The tours of duty of sergeants, roundsmen, patrolmen, (not including detective sergeants) on duty in the open air, on the streets or other public places of a city of the first class or a city of the second class shall be changed at least once in each calendar month. Provided, however, that in cities of the first and second class which on December thirty-first, nineteen hundred and ten, had in force and operation a three platoon system, the tour of duty of such platoons and each member thereof, and the manner of changing such tours of duty may, at the election of the local authorities, having the management, control or direction of the police force, be and continue as the same were in force and in operation on said December thirty-first, nineteen hundred and ten.
N.Y. Police Certain Municipalities Law § 2