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 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”.