The epoch and units
Unix time starts at 1970-01-01 00:00:00 UTC. A timestamp of 0 names that instant, while negative values name earlier instants.
Many systems store whole seconds. JavaScript commonly stores milliseconds, producing values roughly 1,000 times larger. A converter should reveal the chosen unit rather than silently guessing.
UTC versus local display
UTC provides a shared reference. Local time applies a regional offset and daylight-saving rules, so the same timestamp may display as different dates in different locations.
Converting a local wall-clock time back to a timestamp also requires the intended time zone. A browser can interpret the visitor’s current zone, but that is not automatically the zone where an event occurred.
Practical limits
Unix timestamps do not encode a time-zone name, calendar format, or leap second. Historical and future local conversions depend on the time-zone data available to the browser or runtime.