In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Exalted] said:
إِنَّ ٱلۡإِنسَـٰنَ لَفِى خُسۡرٍ
إِلَّا ٱلَّذِينَ ءَامَنُواْ وَعَمِلُواْ ٱلصَّـٰلِحَـٰتِ وَتَوَاصَوۡاْ بِٱلۡحَقِّ وَتَوَاصَوۡاْ بِٱلصَّبۡرِ
By Al-‘Asr (the time). Verily! The human is in loss; except those who believe (in Islamic Monotheism) and do righteous good deeds, enjoin one another to the truth and enjoin one another to patience. [Surah Al-Asr]
The majority of the Qur’aan commentators hold that Al-Asr (Time) is Ad-Dahr (the time period in this world) and this is the statement that is carries more weight based on the sound evidences.
Allaah made an oath by Al-Asr (Time) due to it being the period in which moral lessons and signs occur, for indeed the passing of night and day is under the Decree of the All-Mighty, the All-Knowing (Allaah)- organized to serve the welfare of the universe (and its inhabitants) based on the most perfect order and organization; alternating with one another, sometimes not much difference between them and at other times one takes from the hours of the other; the difference between the light, the darkness, the heat and the cold; the different time periods divided into generations, years, months, days, hours and what is lesser than that! These are all signs (creations) of the Lord [The Exalted] and a Burhaan [i.e. proof that clarify and distinguish between truth and falsehood in everything] from the Baraaheen [proofs] regarding Allaah’s All-Encompassing Perfect Ability and Wisdom (free from all imperfections, faults, deficiencies, shortcomings).
Allaah made an oath by Time and that is the period within which human beings perform deeds for which one will either receive punishment or a good recompense. He gave a notification about Time, which is the Mab’da [i.e. the beginning within which humans perform deeds] and notified the performers of deeds about the Ma’aad [the Hereafter], and that just as He was not unable to initiate the beginning, likewise He is not unable to establish the Hereafter; and that His Wisdom, which necessitated the creation of Time, the performers of deeds and their deeds, and categorized them into good and bad deeds, (i.e. deeds chosen by the person after being given free will and shown guidance through the Messengers) establishes the fact that the people are not equal, and it cannot be that the good doer will not receive a good recompense and that the evil doer will not be recompensed for his evil, and it cannot be that there will not be two groups- the gainers and the losers; but rather the human being in essence is a loser, except the one whom Allaah shows mercy, guides, blesses him with Eemaan, the ability to perform righteous deeds himself and the ability to command others. The opposite of this person is the other one who is reduced to the lowest of the low, except those who believe (in Islamic Monotheism) and do righteous deeds, then they shall have a reward without end (Paradise). [An Excerpt from At-Tibyaan Fee Aqsaam Al-Qur’aan 83-88. slightly paraphrased]