Each FDR sub-frame is matched to the recorder's own "SRN" = Sub-frame Reference Number
(which counts seconds of TIME along the recorder's history).
As an example, the NTSB now offers the "docket" from a mishap, which often includes FDR or CVR records. Consider this mishap in which the CVR was overwritten -- so the recorded data isn't sync'd to local-time nor UCT:
FDR Plots 1
On this exemplar FDR-data time-history, you can see horizontal axis is FDR-SRN
. So, you can create your own time-reference, and count forward or back (seconds of time).
Answer to your question: this FDR doesn't know UTC nor local, this mishap-DFDR doesn't get the feed from captain's clock (not like the FMS time input); this FDR did not record GPS-time, nor ACARS-time. [In most cases, the FDR Gp Leader sync's the FDR to various time sources, recorded on the CVR.]
BTW -- on this exemplar FDR plot, you can see an example of the limited knowledge-base of NTSB's FDR folks: the top-parameter was labeled "Gear WOW" by the NTSB- Recoder-guy. This was an erroneous label, the aircraft type did NOT record Weight-on-Wheels (so the parameter did NOT show Main Landing Gear Touchdown). Instead, the Air-Grnd sensor only recorded NLG touchdown, which happened five-seconds after the LHS-MLG touched-down off the rwy's left edge. This mistake would have confused anybody attempting to correlate the data.
= = = edit, to add FDR-CVR-local sync' example = = =
From a FDR Grp Chrmn's Rpt, pg4 [B737, MDW, 8Dec05]
"... Time Correlation of CVR and FDR
The last five microphone key
ing events recorded by the FDR
and the radio transmission recorded by the CVR
were used to establish a common time base
for the two recordings. ... analysis of the time interval between the start and stop of each radio transmission as recorded by the FDR and CVR indicated a linear relationship ... The Aircraft Performance Group established the following local time correlation...:
Correlation Anchor Point (Local = CVR = FDR)
- - - - - - - Local Time - - - - - || - - - CVR Elapsed Time - - - - || - FDR Elapsed Time
Hr:Min:Sec. - - - Seconds - - || - - Hr:Min:Sec. - - Seconds - -|| - - Seconds
19:13:52.558 - - 69232.558 -|| - - 00:30:18.76 - - 1818.76 - - || - -96396.901