BigInt keeps the source integer exact while Date supplies only the calendar millisecond. EpochLens combines both without converting the full value to Number.
A 19-digit fixture retains nine decimals
1704067200123456789 ns converts to 2024-01-01T00:00:00.123456789Z.
−1 ns uses the preceding second
The exact ISO result is 1969-12-31T23:59:59.999999999Z, proving the floor-division rule.
Copied evidence keeps the unit names
The summary labels UTC ISO, seconds, milliseconds, microseconds and nanoseconds so a bare integer cannot be mistaken for another unit.