(a) For the purpose of establishing compensation rates and the basic rates for vacation and sick leave credit earnings, all professional personnel who previously were employed by a county school system or the public library system in this State and who are appointed to positions in the Department shall be given credit as employees of the Department for the years of service as employees of the county school system or public library system from which they transferred.

(b) For the purpose of establishing vacation and sick leave credit earnings, this section applies to all professional personnel employed by the Department before July 1, 1972.