The timeline between the revelation of first verse of the Quran and the last is around 23 years. Allah says that this book descended on the night of Qadar. How can this be explained?
The whole Quran was descended on the night of Qadr on the heart of the Prophet Muhammad (SAWA) which is known as Inzaal إنزال. After that, Quranic verse were descended in different occasions which is called Tanzeel تنزيل.
This means the the whole Quran was descended on the night of Qadr as Inzaal on the heart of the Prophet, then it was descended again in parts during 23 years in Tanzeel.
'Wassalam.