About the timestamp converter
A Unix timestamp counts the seconds since the first of January 1970, UTC. It is how nearly every system stores a moment in time internally, and it is completely unreadable to a person — which is the entire reason this page exists. You see one in a log line or a database column and need to know when that actually was.
The most common mistake is the unit. Ten digits is seconds; thirteen is milliseconds. Reading one as the other puts you in 1970 or somewhere around the year 56000, and both are obvious once you have been caught by them and invisible until then. The unit is detected here from the magnitude, and the tool tells you which one it used.
Every conversion is shown in UTC and in your local time, side by side, because that is the other place these go wrong. A timestamp has no time zone — it is an absolute instant — but every rendering of it does, and a bug report saying an event happened at nine in the morning is not much use without knowing whose morning.
The day of the week is deliberately shown in UTC rather than locally, so that the same timestamp does not appear to fall on two different days for two different readers. The relative description underneath is the quickest way to sanity-check a value: if a log timestamp reads as being three years from now, the problem is upstream of whatever you were debugging.
Conversion in the other direction takes a date in your local time zone and gives you the timestamp in both units, which is what you usually want when constructing a query against a table of epoch values.