Commentary
The length of the day of Qiyamah
The text in verse 5 cited above says: فِي يَوْمٍ كَانَ مِقْدَارُهُ أَلْفَ سَنَةٍ مِّمَّا تَعُدُّونَ (in a day the measure of which is a thousand years according to the way you count - 5). And it appears in a verse of Surah Al-Ma` arij:... (in a day the extent of which is fifty thousand years - 70:4).
A simple explanation of this is what appears in Bayan-ul-Qur'an - since this day will be horrendous, people would find it very long. Then, this length will be felt in terms of one's measure of faith and deeds. Big criminals will find the day longer while the small ones will find it comparatively shorter, so much so that the day some would feel to be of a thousand years, to others, it would appear as of fifty thousand years.
Tafsir Ruh-ul-Ma’ ani reports several other explanations from the ` Ulama' and Sufiya'. But, all these are nothing but conjectures. Something which can be traced back to a proved meaning of the Qur'an or something which can be believed in is just not there. Therefore, the most sound approach in this matter is the same as was taken by the early elders of Islam, the Sahabah and the Tabi` in: They had left the knowledge of this difference of one and fifty to Allah without explaining it on the basis of human whims and, on their part, they had found it sufficient to say, ` we do not know.'
About it, Sayyidna ` Abdullah Ibn ` Abbas ؓ said:
ھُمَا یَومَانِ ذَکَرَھُما اللہُ تَعَالٰی فِی کِتَابِہٖ اللہُ تَعَالٰی اَعلَمُ بِھِمَا وَاَکرَہُ اَن اَقُولَ فِی کِتَابِ اللہِ مَالَا اَعلَمُ
They are two days mentioned by Allah Ta’ ala in His Book. Allah Ta’ ala knows best about them and I dislike to say in the matter of the Book of Allah that which I do not know.