My .fit file has the wrong date — the FIT time epoch
If your activity claims to be from the early 1990s, the bytes are right and the reader used the wrong starting point for time.
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a .fit file from a watch or bike computer
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FIT counts from 1989, not 1970
Most computing measures time as seconds since 1 January 1970 — the Unix epoch. FIT does not: its timestamps count seconds since 31 December 1989 UTC. Read a FIT timestamp as if it were Unix time and every date lands almost twenty years too early, which is why a ride from last week can show up in 2004 or earlier. The bytes are correct; the reference point was wrong.
A second epoch trap: local vs UTC
FIT timestamps are UTC, but a file may also carry a local-time offset separately. A reader that ignores the offset shows every activity in UTC, shifting your morning ride by your timezone; one that double-applies it shifts twice. Getting a FIT date right means using the 1989 epoch and applying the recorded offset exactly once — two independent steps that each look like a “wrong date” bug when missed.
See the correct time here
This page reads your file on your device with the FIT epoch and the recorded offset, so the timestamps match when the activity actually happened. It runs locally and uploads nothing.
If this was not your problem, the four things that actually break a .fit file covers the rest, and the file reader opens any file whatever is wrong with it.