29 CFR § 553.231 - Compensatory time off.
(a) Law enforcement and fire protection employees who are subject to the section 7(k) exemption may receive compensatory time off in lieu of overtime pay for hours worked in excess of the maximum for their work period as set forth in § 553.230. The rules for compensatory time off are set forth in §§ 553.20 through 553.28 of this part.
(b) Section 7(k) permits public agencies to balance the hours of work over an entire work period for law enforcement and fire protection employees. For example, if an employee engaged in fire protection activities' work period is 28 consecutive days, and he or she works 80 hours in each of the first two weeks, but only 52 hours in the third week, and does not work in the fourth week, no overtime compensation (in cash wages or compensatory time) would be required since the total hours worked do not exceed 212 for the work period. If the same employee in fire protection activities had a work period of only 14 days, overtime compensation or compensatory time off would be due for 54 hours (160 minus 106 hours) in the first 14 day work period.