A Unix timestamp counts seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. Many APIs multiply that count by 1000 and call the result milliseconds.
Which one did you paste?
A current second timestamp has 10 digits. A current millisecond timestamp has 13. The Unix Timestamp Converter treats an integer below 100000000000 as seconds unless you select milliseconds yourself. That rule is stable. It will not silently flip when the clock rolls from 10 digits to 11 in the year 2286, and it will not treat today’s milliseconds as seconds.
If a date lands in the year 50000, you almost certainly decoded milliseconds as seconds. Switch the unit and convert again.
Time zones
The UTC reading does not depend on your location. The local reading uses the IANA zone you select, such as America/New_York or Europe/Rome. The converter does not infer a zone from the number. If a civil time falls in a daylight-saving gap, the reverse conversion says that the local time does not exist instead of inventing a silent shift.