The last days started with Christ’s first coming. How do we know this?

Acts 2:14-17 – “Peter . . . addressed the crowd . . .  ‘let me explain this to you . . . this is what was spoken by the prophet Joel: “In the last days, God says, I will pour out my Spirit on all people”‘” (NIV). Peter said the last days had begun then, and quoted Joel’s prophecy to prove it.

1 Timothy 4:1 – “The Spirit says clearly that in the latter times some will abandon and follow deceiving spirits and things taught by demons” (NIV).  That the “latter times” were going on back then, notice what is said in this context, “If you point this out to the brothers and sisters, you will be a good minister of Christ Jesus . . . Have nothing to do with godless myths” (1 Timothy 4:6,7 NIV).
NIV – Avoid speculations

2 Timothy 3:1 – “There will be terrible times in the last days. People will be lovers of themselves . . . Have nothing to do with such people” (NIV). By saying, “Have nothing to do with such people,” those described in the previous verses, it shows that the last days were going on back then.

Hebrews 1:1,2 – “In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last he has spoken by his Son” (NIV). – The last days obviously began with Jesus’ first coming.

James 5:3, 9 – You’ve stored up treasure in the last days (NKJV); Judge is at the door (second coming), near. See NIV ftn. The last days were going on then.

2 Peter 3:3-5 “ – “You must understand that in the last days scoffers will come, scoffing and following their own evil desires. They will say, ‘Where is this “coming” he promised? Ever since our ancestors died,  everything goes on as it has since the beginning of creation.’ But they deliberately forget that long ago  by God’s word the heavens came into being and the earth was formed out of water and by water” (NIV)
The last days were going on then, because Peter, in present tense says, “they deliberately forget.”

Jude 16-18 – These ungodly people (vs. 4) are doing bad now – vs. 16
vs. 17 – apostles warned about it (Peter)

1 John 2:18 – this is the last hour – why? John writes: “Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrist’s have come” (NIV). Many antichrists have come.
The last days were going on then. This was after the “Jewish system” was destroyed. The “last days” started with Christ’s first coming and, of necessity, must end with his second coming.