The Commissioner of Education may extend the term of the certificate of any person who has served in the armed forces of the United States and whose certificate was in force on the day of induction or the spouse of such person. This extension shall be equal in length of time to the total number of months which intervene between the date of entrance into the military service and the date of discharge therefrom. There shall be no fee for this service. Each person who applies for an extension of the term of his or her certificate shall furnish the Commissioner of Education proper evidence of service in the armed forces and of sound physical and mental health at the time he or she applies for such extension.