The "Official Time Reporting Act" amends federal law to require the Office of Personnel Management (OPM) to submit an annual report to Congress and make it publicly available, detailing the use of official time by federal employees. This report, due by March 31st each year, aims to enhance transparency regarding labor relations activities within the federal government, providing comprehensive data to relevant congressional committees. Each federal agency head is mandated to submit necessary information to OPM by December 31st annually, following OPM's specific guidance. A key provision requires agencies to provide a written explanation if their average aggregate official time rate for a fiscal year increases compared to the previous year. The annual report will include extensive details such as the total amount of official time granted, the average time expended per bargaining unit employee, and the specific types of activities for which official time was used. Furthermore, the report must detail the financial implications, including the total amount paid by the Federal Government to employees granted official time, disaggregated by compensation, fringe benefits, and other expenses. It also requires information on the total amount withheld from employee pay for union dues via payroll systems and descriptions of any government property designated for official time activities, including the fair market value of any free or discounted use. OPM must provide this information both in aggregate and disaggregated by agency, with subsequent reports including comparisons to previous fiscal years, and is tasked with issuing guidance to agencies within 180 days of the Act's enactment to ensure consistent data submission.