Any such law enforcement officer who works overtime during any calendar month shall on the last day of such month file in writing a statement as to his election to accept overtime pay or compensatory leave. In the event such law enforcement officer elects to receive overtime pay, such pay shall be included with his compensation for the next succeeding pay period. If he elects to receive compensatory leave, such leave may be taken at any time during the calendar year in which it is earned with the approval of his supervisor, except during times of emergency. The statement shall be certified by the law enforcement officer and approved by his supervisor specifying the length of time that said officer worked in excess of 40 hours in any one calendar week during the preceding month.