Al-Mu'jam Al-Waseet [The Dictionary]
Volume Two
Page 1111

Translation

(al-yawm [the day]): the time from sunrise to sunset. and - the present time. and from that [meaning - the present time] came in the dear revelation [the Quran] : {This day *al-yawm* have I perfected your religion} [Yusuf Ali, in reference to Quran verse 5:3]. and - (in astronomy): the time it takes the Earth to revolve around its axis, which is twenty four hours.

Phrases between normal () parentheses are in the original text, while the ones between square [] brackets are mine.

As you see, Yawm (day) cannot mean just an indefinite period of time.

Arabic Dictionary
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