17:12
وَجَعَلْنَا ٱلَّيْلَ وَٱلنَّهَارَ ءَايَتَيْنِ ۖ فَمَحَوْنَآ ءَايَةَ ٱلَّيْلِ وَجَعَلْنَآ ءَايَةَ ٱلنَّهَارِ مُبْصِرَةًۭ لِّتَبْتَغُوا۟ فَضْلًۭا مِّن رَّبِّكُمْ وَلِتَعْلَمُوا۟ عَدَدَ ٱلسِّنِينَ وَٱلْحِسَابَ ۚ وَكُلَّ شَىْءٍۢ فَصَّلْنَٰهُ تَفْصِيلًۭا
And We have made the night and day two signs, and We erased the sign of the night and made the sign of the day visible that you may seek bounty from your Lord and may know the number of years and the account [of time]. And everything We have set out in detail.
Translation · Saheeh International
Tafsir
Ibn Kathīr (abridged)
The Night and Day are Signs of the Great Power of Allah
Allah reminds us of the great signs that He created, including the alternation of the night and day, so that people may rest at night, and go out and earn a living, do their work, and travel during the day, and so that they may know the number of days, weeks, months and years, so they will know the appointed times for paying debts, doing acts of worship, dealing with transactions, paying rents and so on. Allah says:
لِتَبْتَغُواْ فَضْلاً مِّن رَّبِّكُمْ
(that you may seek bounty from your Lord,) meaning, in your living and travels etc.
وَلِتَعْلَمُواْ عَدَدَ السِّنِينَ وَالْحِسَابَ
(and that you may know the number of the years and to count.) If time stood still and never changed, we would not know any of these things, as Allah says: