Dear Hamzeh,
Wa alaikum assalam
These are expressions which can also signify continuity.
For example:
"And do not send away those who call upon their Lord at morning (ghudw) and evening (ashiyy)" - 6:52
"And be patient yourself with those who call upon their Lord at morning (ghudw) and evening (ashiyy)..." - 18:28
Of course, this does not mean that one is thus permissible to send away or be impatient with those that call upon their lord in the afternoon or those who summon their Lord at other parts of the day. As I am sure you will agree, this would be an unwarranted, quite preposterous suggestion. This is simply a reference to those who call upon their Lord with regularity and consistency.
Thus the expression above and other similar expressions as we will note below, are used to denote some sort of consistent continuity.
For example, the verse below does not imply that the disbelievers accused the prophet of resorting to 'stories' being read to him only at particular parts of the day.
025:005
'And they say: Fables of the men of old which he has written down so that they are dictated to him morning (bukratan) and evening hours (asil- evening hours - time just before sunset)
Arguably, this is a general expression, accusing the prophet of being subject to story-telling consistently.
We note another example:
013.015
And to God falls prostrate whosoever is in the heavens and the earth, willingly or unwillingly, as do their shadows in the morning and the evening hours (asil- evening hours - time just before sunset).
Shadows are present continuously as long as the sun can cast a shadow. The above does not intend to imply that a shadow will only appear in the morning and once in the evening hours (just before sunset).
In another beautiful verse of the Quran, where God describes His light, there is mention of those houses in which shines with God's light and His name is mentioned in morning and in the evenings which again implies something which is done repeatedly and consistently.
"...therein glorifying Him, in the mornings (ghudw) and the evenings (asil))" 24:36
Another example of such an interpretation can be drawn from verse 19:62 where the Quran speaks about Gardens of Eternity in which the dwellers "...shall have their provision in the early morning (bukr) and evening (ashiyy)". This once again does not mean that they will only find provision in two points of any particular day, but arguably, consistently and repeatedly.
Such expressions are also not restricted to Arabic. I have also been led to believe that such expressions also exist in other languages which draw from Arabic such as 'Urdu'. I understand there is a similar expression 'sub-o-shaam' which also denotes some sort of unceasing continuity.
I hope this helps, God willing
Joseph