Reasoning book pg.234 onwards ( from the JW org website):
Definition: The Bible uses the expression “last days” to refer to the concluding time period leading up to a divinely appointed execution that marks the end of a system of things. The Jewish system with its worship built around the temple in Jerusalem experienced its last days from 33 to 70 C.E. What occurred then was pictorial of what would be experienced in a greatly intensified way and on a global scale at a time when all nations would be facing the execution of judgment decreed by God. The present wicked system of things, which extends worldwide, entered its last days in 1914, and some of the generation alive then will also be on hand to witness its complete end in the “great tribulation.”
And then this...
If Someone Says—
‘How do you know that some future generation won’t fit the prophecy even better than this one?’
‘Matching world events to this prophecy is like matching a fingerprint to its owner. There will not be someone else with the identical print. Likewise, the pattern of events that began in 1914 will not be repeated in some future generation.’