How likely was the first day a 24-hour period?
Length of Creative Days.
At the end of each of the first six creative days ,it was said: "And there was evening and there was morning, the sixth day." This, however was not said of the seveth day that God proceeded to rest from his creative work on the earth.[Gen. 2:1-3]- God's day of rest. More than 4.000 years later Paul indicated that the seventh day- God's day of rest was still in progress when he said that Christians should do their utmost to ender it -Vs 11 Hebrews 4:1-11 read on the link below.
https://www.biblegateway.com/passage/?search=Hebrews+4%3A1-11&version=NASB;NET
That a day can be longer than 24 hours is indicated by Genesis 2:4, which speaks of all the creative periods as one “day.”
Genesis 2:4 ESV The Creation of Man and Woman of the heavens and the earth when they were created,in the day that the Lord God made the earth and the heavens.
Also indicative of this is Peter’s inspired observation that “one day is with Jehovah as a thousand years and a thousand years as one day.” (2Pe 3:8 NIV) " But do not forget this one thing, dear friends: With the Lord, a day is like a thousand years, and a thousand years are like a day."
That a day can be longer than 24 hours is indicated by Genesis 2:4, which speaks of all the creative periods as one “day.”
Genesis 2:4 NASB " This is the account of the heavens and the earth when they were created, in the day that the Lord God made the earth and heaven."
Ascribing not just 24 hours but a longer period of time, thousands-billions of years, to each of the creative days better harmonizes with the scientific evidence found in the earth itself. Scientists estimate that the earth is 4.54 billion years old.{Source Google}