The context of Luke 21:22 clearly points to the first century.

Jesus was speaking to His disciples about the destruction of Jerusalem and the Temple (Luke 21:6), warning them of what they would see (Luke 21:20, "when you see Jerusalem surrounded by armies…"). He then says, "For these are days of vengeance, so that all things which have been written will be fulfilled" (Luke 21:22).

That phrase connects the Old Testament prophetic warnings with the judgment coming on that generation (Luke 21:32). It was about Israel, not some future global event. The destruction in 70 AD fulfilled this, not some yet-to-happen twenty-first century catastrophe.

If it's about the twenty-first century, then Jesus lied to them about what they would witness. But if it's about the first century, then prophecy was fulfilled exactly as He said.

So, the only honest answer is: the first century.