An oath by the time.
Allah (swt) swears by what He (swt) wills to what He (swt) wills, because He (swt) is the creator of everything, Has knowledge of everything and their secrets, but, as for us, we do not know the greatness of the things neither their seriousness since we are ignorant of the creations around us.
The oath in the Holy Qur’an is either in affirmative form as this verse, or in negative form as in:
﴾لَا أُقْسِمُ بِيَوْمِ الْقِيَامَة﴿
(I do not swear by the Day of Resurrection.) (Al-Qiyamah: 1)
The purpose in both is one, which is to confirm what is sworn to. However, in the affirmative oath there is a suspicion which Allah (swt) wants to remove, whereas, in the negative oath, there is no suspicion, it is a very clear matter that does not need to be sworn to, but if He (swt) had willed to swear, He would have sworn by that.
In this verse, Allah (swt) swears by the time (of the afternoon) to confirm what is followed in the next two verses.
What is the relation between the time and what is sworn for?
It is said that: The time is an approach for the following ruling which is: The man is always in loss. The time can refer to a point of time or to an idiomatic meaning, which is the prayer of the afternoon; the midst prayer which is mentioned in this verse:
﴾حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلَاةِ الْوُسْطَىٰ وَقُومُوا لِلَّهِ قَانِتِينَ ﴿
(Maintain with care the [obligatory] prayers and [in particular] the middle prayer and stand before Allah, devoutly obedient.) (Al-Baqara: 238)
Moreover, the time can refer to a specific period such as when we say: The Ab-Bassy period, The Umayad period, Atom period and Technology period, hence it has certain conditions and persons.