Instant.add() does not cross the epoch correctly.

[kolton-musgrove]

    describe('cross epoch in seconds', () => {
      const one = instantFns.subtract(inst, { hours: 240, seconds: 1 })
      const two = instantFns.add(inst, { hours: 240, seconds: 1 })
      const three = instantFns.subtract(two, { hours: 480, seconds: 1 })
      const four = instantFns.add(one, { hours: 480, seconds: 1 })

      it(`instantFns.subtract(${inst}), { hours: 240, seconds: 1 }) = ${one}`, () =>
        assert.equal(one, '1969-12-15T12:23:44.678Z'))
      it(`instantFns.add(${inst}, { hours: 240, seconds: 1 }) = ${two}`, () => assert.equal(two, '1970-01-04T12:23:46.678Z'))
      it(`instantFns.subtract(${two}, { hours: 480, seconds: 1 }) = ${one}`, () => assert(instantFns.equals(three, one)))
      it(`instantFns.add(${one}, { hours: 480, seconds: 1 }) = ${two}`, () => assert(instantFns.equals(four, two)))
    })

The preceding code generates timeshifts that are off by days, hours, minutes, and seconds when it should only change the day value be a single increment and the seconds by 1.

Output for the first asser.equal():

• actual - expected

• '1969-12-03T22:37:05.678Z'

• '1969-12-15T12:23:44.678Z'