Epoch seconds or milliseconds in, readable dates out — in your timezone and in UTC.
A Unix timestamp counts the seconds elapsed since midnight UTC on 1 January 1970. Most systems use seconds, but JavaScript, Java and many logging tools use milliseconds, so a value above roughly 100 billion is treated as milliseconds and anything below it as seconds.
Dates are rendered with your browser's own locale and timezone data, which is why the local line matches your clock while the UTC line does not. The ISO 8601 line is the form to use in APIs and databases, since it is unambiguous about offset.
Count the digits. A present-day timestamp in seconds has ten; in milliseconds it has thirteen. The unit actually used is stated under the results.
Because a timestamp has no timezone — it is a single instant counted from a fixed point in UTC. Only its human-readable rendering changes.
Systems storing the timestamp in a signed 32-bit integer overflow on 19 January 2038. Modern platforms use 64-bit values, which last longer than the sun.
Yes, as negative numbers. Type -86400 to get 31 December 1969.