﴿ 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 established the night and the day as two symbols; and thereupon We have effaced the symbol of night and set up [in its place] the light giving symbol of day, so that you might seek to obtain your Sustainer's bounty and be aware of the passing years and of the reckoning [that is bound to come]. For clearly, most clearly, have We spelt out everything!”

(And we appoint the night and the day two portents) two signs; the reference here is to the sun and the moon. (Then We make dark the portent of the night) the light of the sign of the night, i.e. the moon, (and We make) and leave (the portent of the day) i.e. the sun (sight giving) bright and clear, (that ye may seek bounty from your Lord) the bounty of this worldly life and that of the Hereafter, (and that ye may know) and that you may know through the increase and decrease of the phases of the moon (the computation of the years and the reckoning) how to count days and months; (and everything) the lawful and the unlawful, the commands and prohibitions (have We exposited with a clear expositing) We have clearly explained it in the Qur'an.