Hebrews 1:2 says, "in these last days has spoken to us in His Son." That means the writer of Hebrews understood they were living in the "last days" at that very time, not some distant future.

This matches what Peter said in Acts 2:16–17, quoting Joel: "In the last days... I will pour out My Spirit." Peter told the crowd that this prophecy was being fulfilled right then at Pentecost.

Paul also wrote in 1 Corinthians 10:11 that "the ends of the ages have come" upon them.

So according to scripture:

The last days began when Jesus appeared and the gospel was preached.

The Old Covenant was ending, and the New Covenant was being established.

The destruction of Jerusalem in AD 70 marked the culmination of those last days, ending the Old Covenant age completely.

The last days were not about the end of the physical world, but the end of the covenant age and the Hebrews writer saw himself right in the middle of that transition.