How long was a day at the creation of Earth?

... each day is 1 second longer every about 1.5 years

That figure is way off.

According to this Scientific American article, the Earth's rotation rate just after the collision that formed the Moon was about once every 6 hours. At that time, the Moon would have been about 25,000 kilometers away. The tidal effect of the Moon is the major reason the day has been lengthening, and the Moon's orbit has been widening.

The collision is believed to have taken place about 4.5 billion years ago, not long after the formation of the proto-Earth.

There are still some open questions about the impact hypothesis (see the linked Wikipedia article), so this is uncertain.

I strongly suspect that the impact would have erased any information about the Earth's rotation rate before the impact. (It might be possible to estimate the pre-collision rotation rate by modelling the initial formation of the Earth; I don't know whether there's been any research in this area.)

Actually we only gain 1.3 milliseconds every 96-100 years, not 1 second every 1.5 years! :) the shortest known Earth day was 6 hours and the longest is 24 hours & 2.5 milliseconds (today's current day), in 1820 the day was exactly 24 hours, but since it's been nearly 200 years we've gained 2.5 milliseconds to our day. So the days get longer just very shortly, I believe it'll be 15 minutes longer in 50 million years.