The Last Days began with the outpouring in Acts, and is still ongoing (unless you believe the "last days" has ended and we're in some new age, or "that great and notable day of the Lord" has already come??).The fulfillment was acknowledged of His apostles in the acts.
Acts 2:16-21 (KJV) ¯¯¯¯¯¯¯¯¯¯ ¹⁶ But this is that which was spoken by the prophet Joel; ¹⁷ And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: ¹⁸ And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy: ¹⁹ And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke: ²⁰ The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come: ²¹ And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved.
Verse 39 says that the promise was to them, to their children, and to "all that are afar off", which in context, means future generations.
“For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.” — Acts 2:39 (KJV)