The “day of the Lord” is often used in the OT as national judgements, and written in hyperbolic world ending language. So I see no reason, to understand the NT uses any differently.
1.) the “day of the Lord” = God using Babylon to judge Egypt.
Ezekiel 30:2-4, 9-10
Wail, ‘Alas for the day!’
3For the day is near,
the day of the LORD is near; it will be a day of clouds, a time of doom for
a the nations.
4A sword shall come upon Egypt, and anguish shall be in Cush,when the slain fall in Egypt,and her wealth
b is carried away,and her foundations are torn down.
On that day messengers shall go out from me in ships to terrify the unsuspecting people of Cush, and anguish shall come upon them on the day of Egypt’s doom;
e for, behold, it comes!“
Thus says the Lord GOD:
“I will put an end to the wealth of Egypt,
by the hand of Nebuchadnezzar king of Babylon.
notice this “past event” is referred to as a day of the Lord, counter to your argument.
Jeremiah 46:2, 10
2About Egypt. Concerning the army of
Pharaoh Neco, king of Egypt, which was by the river Euphrates at Carchemish and which Nebuchadnezzar king of Babylon defeated in the fourth year of Jehoiakim the son of Josiah, king of Judah:
10That day is
the day of the Lord GOD of hosts,
2.) the “day of the Lord” = God using the meads to judge Babylon
Isaiah 13:1, 6, 17
1The oracle
concerning Babylon which Isaiah the son of Amoz saw.
6Wail,
for the day of the LORD is near;
as destruction from the Almighty
cit will come!
17Behold,
I am stirring up the Medes against them,who have no regard for silverand do not delight in gold.
3.) the day of the Lord = God using Assyria to judge Israel, and exile them.
Amos 5:20, 27
Is
not the day of the LORD darkness, and not light,and gloom with no brightness in it?
27and
I will send you into exile beyond Damascus,” says the LORD, whose name is the God of hosts.
4.) the day of the Lord = God sending babylon to judge judah.
Zephaniah 1:4,7, 12
“
I will stretch out my hand against Judah
and against all the inhabitants of Jerusalem;
7Be silent before the Lord GOD!
For the day of the LORD is near;
At that
time I will search Jerusalem with lamps, and I will punish the men
who are complacent,
That all being said, i still have no idea where you are getting that the day of the Lord being = to the entire gospel age…..