N.Y. PCM Law § 2

§ 2. The tours of duty of sergeants, roundsmen, patrolmen, (not\nincluding detective sergeants) on duty in the open air, on the streets\nor other public places of a city of the first class or a city of the\nsecond class shall be changed at least once in each calendar month.\nProvided, however, that in cities of the first and second class which on\nDecember thirty-first, nineteen hundred and ten, had in force and\noperation a three platoon system, the tour of duty of such platoons and\neach member thereof, and the manner of changing such tours of duty may,\nat the election of the local authorities, having the management, control\nor direction of the police force, be and continue as the same were in\nforce and in operation on said December thirty-first, nineteen hundred\nand ten.\n