Unix Timestamp Seconds vs Milliseconds

Step-by-step

1. Check the documentation for the system producing the timestamp.
2. Inspect the number of digits as a quick clue, not as the only test.
3. Convert the value using the correct unit.
4. Compare the result with a known date.

Why the difference matters

A value measured in milliseconds is 1,000 times larger than the same instant represented in seconds. Using the wrong unit therefore produces a dramatically different date.