What Unix epoch time is
Unix time (you'll also see it called POSIX time) is just a count of seconds. The clock starts at midnight UTC on January 1, 1970 — the "epoch" — and a timestamp is how many seconds have gone by since. Anchoring it to UTC means the same number points to the same instant everywhere, with no time zones to juggle. That's why so much software leans on it.
Timestamp to date
Drop a Unix timestamp into the "Timestamp to Date" box to read it as an ordinary date and time. You get the result in UTC and in your own local time, side by side. "Auto" works out whether the number is seconds or milliseconds, but you can force either unit if you'd rather be explicit.
Date to timestamp
The "Date to Timestamp" box runs it backwards. Pick a date and time, say whether you mean local or UTC, and you get the epoch value in both seconds and milliseconds — the number you'd drop into an API call, a database row, or a cron job.
Seconds or milliseconds
This is what trips people up. Ten digits is seconds; thirteen digits is milliseconds, at a thousand per second. Unix tools and most databases talk in seconds, while JavaScript works in milliseconds. If a timestamp converts to some date in the year 50,000 or back before you were born, the odds are you've got the units crossed.
What date does a giant timestamp land on?
People paste in a huge number all the time just to see where it falls. The classic is 9999999999 — ten nines, read as seconds — which comes out to November 20, 2286, more than 95,000 days off. Its thirteen-digit form, 9999999999999, gives the same date, since auto-detection catches the unit. Two others that get searched a lot: 2000000000 is May 18, 2033, and 1234567890 is February 13, 2009 — the second Unix time rolled over to those neat digits, which people genuinely stayed up for.
Why it's everywhere
One integer, no room for confusion. It sorts, it compares, it stores as a plain number, and it doesn't care which time zone the server sits in. Logs stamp events with it, APIs pass it around, schedulers fire on it. And doing math on it — add an hour, measure the gap between two events — beats parsing formatted date strings every time.
