5 C.F.R. § 550.1638
An agent who uses official time under 5 U.S.C. 7131 may be assigned to a Level 1 or Level 2 regular tour of duty, but is required to perform agency work during obligated overtime hours or to accrue an overtime hours debt. Official time may be used during overtime hours only when, while the agent is engaged in the performance of agency work, an event arises incident to representational functions that must be immediately addressed during the overtime hours. CBP may excuse the agent from duty during scheduled obligated overtime hours if it determines that an agent's official time duties during the basic workday make it impracticable to perform agency work during the scheduled obligated overtime hours on that day. The agent will accrue an overtime hours debt for that excused time. If CBP excuses the agent in this manner, then it must provide the agent with an opportunity to eliminate the resulting overtime hours debt by performing agency work outside the agent's regular tour of duty at another time. As provided in § 550.1621(e) , official time during regular time is considered to be "work" when an agent otherwise would be in a duty status in applying paragraphs (a)(3) and (b)(3) of § 550.1621 .
5 C.F.R. §550.1638