The correct lights off start time of a race & total time for race session

[afya]

I am having trouble getting the correct lights off start time and subsequently the total time of a race session.

Take 2026 China for example. The total time for ANT is 01:33:15.607. This is correctly shown from seesion.results

However, session.laps.pick_laps(56) gives 02:29:39.883000. Subtracting session_start_time 0:56:24.050000 from it, it gives 01:33:15.833.

I also tried getting the start time from session.laps.pick_laps(1) [Time] - [LapTime] which gives 00:56:24.312000 and total time from lap 56 becomes 01:33:15.571

Summing up [LapTime] is also way off, I assume it is due to some laps not accurate.

[LiudengZhang]

From what I can tell, the ~200–300 ms discrepancy is expected. session.results['Time'] comes from the official FIA post-race results and should be the authoritative value, while laps['Time'] is reconstructed from the live timing stream — the docs note it probably should have been named "SessionTime."

The gap seems to come from cross-stream jitter (session_start_time and laps['Time'] originate from separate API feeds that aren't perfectly synchronized) and from Fast-F1's post-processing that refines lap timestamps by comparing sector triggers. The time_explanation doc page mentions that reconstructed timestamps can't be verified to millisecond precision. For authoritative race time, session.results['Time'] should be the one to rely on. Summing LapTime drifts further because individual laps can be NaT or inaccurate for pit laps and restarts.