Exactly when does a day start?

Question

If I say midnight Sunday am I referring to the time between Saturday and Sunday or Sunday and Monday. In other words does a day start at "exactly" midnight or an instant later?

Answer 1

midnight is the "0" of the day, like the 0 on a tape measure.

Whether you consider this the start or a tiny bit later is up to you.

Answer 2

I'm retired.

I don't do nuthin', and I don't start before noon.

Answer 3

An instant later.

Answer 4

Officially, at 12:00 AM.

Colloquially, when the sun rises.