A "61.58" refers to the Pilot-in-Command (PIC) proficiency check required by FAA regulation 14 CFR § 61.58 for pilots operating aircraft that require more than one pilot or are turbojet-powered. This proficiency check is valid for 12 calendar months. Specifically, the regulation states that no person may act as PIC of such an aircraft unless they have completed a proficiency check within the preceding 12 months. Additionally, there is a requirement to complete a proficiency check in the particular type of aircraft every 24 calendar months. The FAA provides a "grace month" provision: if you complete the check in the month before or the month after it is due, you are considered to have taken it in the month it was due for the purpose of calculating the next deadline. This ensures pilots maintain high standards of safety and operational knowledge in complex turbine aircraft. If the 12-month period lapses, the pilot is no longer "current" and cannot legally act as PIC until a new proficiency check is satisfactorily completed with an authorized examiner or instructor.