instant

An "instant", or "exact time", is a point in time without regard to any time zone: just the time since a particular external reference point, often called an "epoch".

Here, the epoch is 1970-01-01T00:00:00Z, also known as POSIX's Seconds Since the Epoch, also known as Unix Time.

Note that even if the seconds field is negative, incrementing nanoseconds always represents moving forwards in time. For example, { -1 seconds, 999999999 nanoseconds } represents the instant one nanosecond before the epoch. For more on various different ways to represent time, see https://tc39.es/proposal-temporal/docs/timezone.html