For the purpose of applying the provisions of this Chapter, employment in the Executive, Legislative or Judicial branch of State service in positions not subject to this Chapter shall be credited as State service; provided, however, that except as otherwise provided by law, leave credits may not be transferred upon movement from such positions to positions subject to this Chapter except where such credits were earned and accumulated in accordance with attendance and leave regulations which are substantially equivalent to this Chapter, and there is a written agreement between the President of the Civil Service Commission and the agency wherein such credits were earned and accumulated governing the transfer of leave credits upon such movements. Other public employment not subject to this Chapter may be credited as State service for purposes of this Chapter, provided such employment was subject to attendance and leave regulations substantially equivalent to this Chapter, and provided there is a written agreement between the President of the Civil Service Commission and the public agency wherein such employment occurred governing the crediting of such employment as State service and the transfer of leave credits upon movement of employees to and from such agency and positions subject to this Chapter.