Two causes:

Battery removal. Most trail cameras store time and date in volatile RAM backed by the main battery. When the battery is fully depleted and removed, the clock resets to factory default (often January 1, 2000). Always reset date and time after replacing batteries or recovering from complete battery drain.

Time zone mismatch. If the camera was set up in one location and deployed in a different time zone, or if the clock was set incorrectly during initial setup, all timestamps will be offset by a fixed amount. Re-enter the correct time zone and current date/time in camera settings.

For cameras with GPS functionality, some models can automatically sync time from GPS satellite data — this eliminates manual clock management entirely.