The Torah says the world was created in 6 days. And that Rashi says explicitly that when the Torah says Vayehi Erev Vayehi Voker Yom Echad it means 24 hours.
The 6 days of creation were in fact 24 hours. How could they not be? Aren’t days 24 hours now? So when did this change? Where does it indicate in the slightest that the first Sunday after creation (or the first Shabbos?) was suddenly shorter than previous days?
On the contrary – it’s clear that on the fourth day Hashem said the sun should shine during the time-period that was called “day” and the stars/darkness should rule during the time-period called “night”. Since then, that hasn’t changed, and obviously as we can see today, the sun and the stars have decided that the time period called day plus the time period called night, are 24 hours.
The Gemora says this explicitly. 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”.