The Torah uses the same word יום in the same verse to denote both "daylight" and "24 hours":
וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם וְלַחֹשֶׁךְ קָרָא לָיְלָה
וַיְהִי־עֶרֶב וַיְהִי־בֹקֶר יוֹם אֶחָד׃
God called the light Day, and the darkness He called Night. And there was evening and there was morning, a first day.
Those words are NOT interchangeable: it is meaningless to change them: "God called the light 24 hours" or "there was evening and there was morning, the first light".
I know they didn't have the word יממה yet, but why does Torah use the same word to denote both?