Ecmascript internally represents date/time as the unix timestamp. That's basically the number of seconds since 1970, also called the Unix Epoch. Ecmascript tracks this on a millisecond basis.
If a time value is NaN, it represents no date.
Note that Date ignores leap seconds (one second every 4 leap years). It assumes every day is exactly 86400000 milliseconds.
Ecmascript dates could represent about 285616 years in either direction from 1970.
The specification only actually supports 100.000.000 days in either direction.
The exact epoch, 1 January, 1970 UTC, is represented as +0.