Testing dates before 1970 in JavaScript

The bug: negative milliseconds round toward zero

new Date(-1500) is 1969-12-31T23:59:58.500Z, one and a half seconds before the epoch. Math.trunc(-1.5) is -1, not -2, and -1500 % 1000 is -500, not 500. Code that splits a timestamp into seconds and a fraction with truncation and % gets 23:59:59 and a negative fraction.

A naive implementation that fails

Protocol Buffers and many APIs encode time as { seconds, nanos } where nanos must be between 0 and 999,999,999:

function toTimestamp(date) {
  const ms = date.getTime();
  return { seconds: Math.trunc(ms / 1000), nanos: (ms % 1000) * 1e6 };
}

toTimestamp(new Date(-1500)); // { seconds: -1, nanos: -500000000 }  invalid

The edgegoat test that catches it

import { expect, test } from 'vitest';
import { dates } from 'edgegoat';

test.each(dates({ tags: ['pre-1970', 'epoch'] }))('$label', ({ value }) => {
  const { seconds, nanos } = toTimestamp(value);
  expect(nanos).toBeGreaterThanOrEqual(0);
  expect(nanos).toBeLessThan(1e9);
  expect(seconds * 1000 + nanos / 1e6).toBe(value.getTime());
});

Local mean time before time zones existed

Before standard time zones were adopted, each city kept its own local mean time. The tz database records offsets with seconds, such as -03:06:28 for Sao Paulo before 1914. getTimezoneOffset() reports whole minutes, so local arithmetic on old dates can drift by seconds. The lmt case in dates() covers it.

Year 0, negative years and the far edges

dates() also includes year 0, year -1 (-000001 in ISO format), the minimum and maximum valid dates (plus and minus 8.64e15 ms), year 9999 and year 10000, where toISOString() switches to a six digit, signed year.

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