Allah is the Creator and Controller of the Universe
Allah tells us that He is the Creator of all things. He created the heavens and earth and all that is between them in six days, then He rose over the Throne -- we have already discussed this matter elsewhere.
مَا لَكُمْ مِّن دُونِهِ مِن وَلِيٍّ وَلاَ شَفِيعٍ
(You have none, besides Him, as a protector or an intercessor) means, only He is the Sovereign Who is in control of all affairs, the Creator of all things, the Controller of all things, the One Who is able to do all things. There is no Creator besides Him, no intercessor except the one to whom He gives permission.
أَفَلاَ تَتَذَكَّرُونَ
(Will you not then remember) -- this is addressed to those who worship others apart from Him and put their trust in others besides Him -- exalted and sanctified and glorified be He above having any equal, partner, supporter, rival or peer, there is no God or Lord except Him.
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.'
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.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 3–8
On the face of it, this is an ordinary statement consisting of just a few words. But this is a very serious and portentous claim, which in the entire history of religion, nobody has had the courage to profess, except those chosen few to whom the Book of God was in actuality revealed. Other than the appointed person, anyone who mustered the audacity to utter these words, was either a trifler or a madman. The Quran is a proof in itself. Its miraculous style, its claims that have not been proved wrong even after centuries have elapsed,—all these and similar considerations prove that it is a book sent by God. And since it is a book of God, it is absolutely necessary that everybody should pay heed to its warnings and gives serious thought to them.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.
