From the FAA
Regulatory and Guidance Library:
Code of Federal Regulations
Part 121 OPERATING REQUIREMENTS: DOMESTIC, FLAG, AND SUPPLEMENTAL OPERATIONS
Appendix B--Aircraft Flight Recorder Specifications
Sec. B121.1
Appendix B--Airplane Flight Recorder Specification.
Parameter: Time (GMT or Frame Counter) (range 0 to 4095, sampled 1 per frame).
Range: 24 hours------------
Accuracy sensor input to DFDR readout: ± 0.125% Per Hour-------------------
Sampling interval (per second): 0.25 (1 per 4 seconds).
Resolution readout4: 1 sec.
4This column applies to aircraft manufactured after October 11, 1991.
Amdt. 121-197, Eff. 10/11/88