[UCANSEE2]

you can define a day to have the duration of a single rotation of a planet that does not exist yet.

That's pretty much a standard all over the entire Universe, and was from 'day one'.

Problem is, the 'length' of the day is radically different for each planet. Even in our own solar system.

[Fichori]

“Problem is, the 'length' of the day is radically different for each planet. Even in our own solar system.” [excerpt]

God didn't say Six Martian days.

Methinks the mole in thy yard hath pushed up an Everest or two.