Almost every timestamp you have ever seen in a log file, a database row, or an API response is, underneath, the same thing: a count of seconds. Unix time is the number of seconds that have elapsed since 1970-01-01T00:00:00Z, a moment known as the epoch.
One number, no time zone
The defining feature of Unix time is that it is a single integer with no time zone attached. The instant "1700000000" is the same instant everywhere on Earth; it is only when you want to display it to a human that a time zone enters the picture. That property is exactly why it is so widely used: two systems on opposite sides of the world can compare, sort, and store timestamps without ever negotiating a zone. A larger number is always later; subtraction gives a duration in seconds.
Before the epoch
Time did not begin in 1970, so Unix time simply goes negative for earlier moments. -86400 is exactly one day before the epoch, 1969-12-31T00:00:00Z. Most systems handle negative timestamps cleanly, though some older code assumed time was unsigned and breaks before 1970.
Converting to a date is arithmetic
Turning a Unix timestamp into a calendar date needs no clock and no network — it is pure arithmetic over the proleptic Gregorian calendar. You divide and modulo your way from seconds to days, account for leap years, and arrive at a year, month, day, and time. Because the rules are fixed, the same timestamp always yields the same date, which is what lets a converter like this one run entirely in your browser and produce identical output every time.
Why it won
Unix time beat the alternatives for boringly practical reasons: it is compact (one number), trivially comparable, lexically irrelevant (no string parsing to compare two instants), and unambiguous (no "is this 3 PM in London or Tokyo?"). The cost is that the raw number is unreadable to humans — 1700000000 means nothing at a glance — which is the entire reason date formats and converters exist.
Two subtleties are worth knowing before you trust a raw timestamp: which unit it is in (seconds, milliseconds, or smaller), and the fact that Unix time quietly ignores leap seconds. Both have their own articles.