An eligible employee shall elect to: join a pension or retirement system in accordance with the provisions of this Division II applicable to that system; or participate in the program. An eligible employee shall: make an election under this section in writing; and file the election with the Board of Trustees and the employing institution at commencement of employment. An eligible employee’s election under this section is a one–time, irrevocable election. The effective date of the election shall be the day of commencement of employment.