Watchtower establishes that their version of applying the Bible, particularly Revelation, is a fact. Those facts prove to the flock that Watchtower was appointed by God Almighty.
The calendar continues to force Watchtower to dissect their version of applying the Bible until nothing is left of their original version that established their facts and demonstrated their appointment.
So they stick with the thinnest of connections to their original version by references to 1914 and the undeniable fact that World War I broke out that year. Despite the fact that Watchtower never said this was what would happen in 1914, they have managed to keep that going.
Their tie to their original version has stretched and is now just a thread. That "generation" has passed away and the new light about "overlapping" just isn't holding people. Doomsday cults need a doomsday in sight. Before they lose all their members, I see them coming out with a new Doomsday calendar that will place Armageddon far enough into the future that all the current GB members will be dead or ancient by the time it arrives.
I think they have seriously considered 2034 (120 years since 1914 just like 120 years declared to Noah before the flood) but have not been able to do that all at once, gauging reaction to changes they bring about. They may need to buy a little more time and connect the 120 years to something past 1914. We will see.