Allah (swt) says: This matter is very clear and does not need any swear and if I were to swear, I would swear by it.

بِالشَّفَقِ (twilight glow): It is the redness that remains on the horizon after sunset, or it is the whiteness that follows that redness. Mujahid said that it refers to the whole day.

However, it is possible that the whole day is meant by this oath even if just a part of it is mentioned.