Days created on the fourth day?

If the sun was created on the fourth day, as Genesis claims, how could it be the fourth day?

A day, by definition, is one revolution of the earth.

But a day's status as a day is meaningful only because of the alternation of light and darkness, as Genesis 1:5 acknowledges.

Further, if light did exist from day one, prior to the creation of the sun, moon and stars on day four, what was its source? And how did its light create the alternation of light and darkness to distinguish the days from nights? Did it switch off for a period every 24 hours?

If God was the source of this light, why did he bother? Why not just create the sun before the days?

Surely this contradiction shows that the Genesis creation account is non-literal mythology written by people whose understandable ignorance of science has resulted in an untrue account. Do you agree?