An anonymous day

Charles sat by a river in the Sudan, waiting to advance.

He watched two crocodiles swim past, and heard one say to the other “I keep thinking its Tuesday!”

Charles was in a contemplative mood and thought “why Tuesday”?

“I know why there are 365 or 366 days in a year, but why are there seven days in a week, twelve months in a year?”

“Why sixty seconds in a minute, sixty minutes in an hour, 24 hours in a day?”

“The answer must come from somewhere near here, where civilisation started.”

He could not find an answer.

Can you?