Values before the Unix epoch are negative. Sub-second conversion must use floor division so the remainder stays a non-negative fraction within the preceding second.
−1 ns is not epoch plus a negative fraction
It is 1969-12-31T23:59:59.999999999Z. A truncating quotient would produce the wrong calendar second with a negative remainder.
Signed integers remain exact
The minus sign applies to the complete epoch quantity. Formatting derives a human decimal only after the exact integer is parsed.
Receiving systems may reject pre-epoch time
A mathematically valid timestamp does not prove a database, protocol or language accepts it. Check the receiving contract separately.