In assigning serial definitions to the stratification of lunar regolith with precise ETA of first impact; what year, month, day and time did the heavy bombardment period begin?
What makes you think the length of a day on Earth has been constant since the formation of the Earth?
The length of an Earth day has been increasing slowly throughout most of the Earth's 4.5-billion-year history, says Dr Rosemary Mardling, mathematical scientist at Monash University, and it all has to do with the Moon.
"The reason is that the Moon is attempting to slow down the spin of the Earth. The Earth was spinning very much faster when the Moon was formed," says Mardling.
Back when the Moon was formed the length of an Earth day was a very brief two to three hours, and a much closer Moon was orbiting the Earth every five hours.
... the Earth slows down a little bit and the Moon moves away from the Earth," says Mardling.
"... The spin down rate is very slow," says Mardling, "It's about two milliseconds per century. So the Earth's day is getting longer by a 500th of a second every century"
... Since the dinosaurs lived during the Mesozoic era, from 250 million years ago to 65 million years ago, day length would have been longer than 21 hours and probably closer to 23 hours.
Given that the length of a day on Earth has never been constant throughout its history it would be totally impossible to assign a year, month day or time of day for the commencement of the heavy bombardment period or any other event in ancient geologic history.