“How does the fact that Hashem said the sun should shine in the day and the stars at night prove that Hashem created the sun and stars in literally 24 hours?”
646 –
The Gemora says this expicitly. It describes 10 things that were created on the first day of creation, one of which is the “length of the day and night” – as it says, “vayehi erev vayehi voke yom echad”. So the time span of the day was created on the first day of creation. And, as Rashi states, it means “[the day and night together] – i.e. 24 hours between them”.