A. Any teaching hospital affiliated with an institution of higher education, other than the Virginia Commonwealth University Health System Authority or the University of Virginia Medical Center, may establish a retirement plan covering in whole or in part its employees who are health care providers, as determined by the Department of Human Resource Management pursuant to § 2.2-2905, and is authorized to make contributions for the benefit of its employees who elect to participate in such plan or arrangement rather than in the retirement system established by this chapter. Any such alternative retirement plan shall not become effective until July 1, 1991, or any time thereafter, as determined by such teaching hospital. Any health care provider employed by such teaching hospital on or after July 1, 1991, may make an irrevocable election to participate in either the retirement plan established by this chapter or the plan provided by the teaching hospital, in accordance with guidelines established by the Virginia Retirement System. The election herein provided shall, as to any health care provider employed after the alternative retirement plan implementation date, be exercised not later than thirty-one days from the time of entry upon the performance of his duties.