Mark 13:24-27
"But in those days, after that tribulation, the sun will be darkened, and the moon will not give its light, and the stars will be falling out of heaven, and the powers that are in the heavens will be shaken. And then they will see the Son of man coming in the clouds with great power and glory. And then he will send out the angels and will gather his chosen ones together from the four winds, from earth’s extremity to heaven’s extremity."
These things haven't happened yet.
If the last days were in the first century, then these things would have happened by now, because it says "in those days, after that tribulation".
There was tribulation in the first century, but Jesus was also talking about a future tribulation that would be greater.
The phrase "last days" can be applied to many different scenarios, as already brought out (ie, "the last days of August"). There were "last days" of Jerusalem in the first century. There were "last days" before the Flood of Noah's day. There were "last days" for Sodom and Gomorrah.
Revelation shows that when the "last days" of this entire system of things begin, religious institutions will be plagued and destroyed. When Jesus takes the throne in heaven and begins his ride, as depicted by the first horse, he will clear out the heavens of God's enemies and also allow the attack on religious institutions in order to clear God's name of reproach. That hasn't happened yet. The "last days" culminating in the final battle of Armageddon haven't started yet. Once the "last days" start, they won't be hundreds of years long. They will be "3 1/2 times", which Revelation shows is the same as 42 months, also called in Revelation "a short period of time".