Days getting longer.
The rotation of Earth is slowing down.
Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to other, distant, stars. Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern-day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BCE.
Wikipedia, Earth's Rotation, 2019
The rotation of Earth is slowing down, that is, days were shorter in the past; but the few milliseconds per century would have been impossible to detect 1400 years ago. However the Quran said that the days are getting longer:
[Quran 7.54] And your Lord, Allah, who created the Heavens and the Earth in six days and then settled on the Throne. [Allah] Covers the night with the day, asks for it persistently; and the sun and the moon and the stars enslaved by His orders. Is this not His creation and His command? Blessed be Allah the Lord of all the worlds.
"Yatlubuhu hatheethan يَطْلُبُهُ حَثِيثًا" means asks for it persistently; more of the day and more of the night. If God asks more of the day and more of the night then this means the days are getting longer.
How could an illiterate man who lived 1400 years ago have known that the days are getting longer?