All the pieces to this so-called prophecy were penned long before the fourth century when Hillel II establish a fixed calendar. So, I call bunk on that. The Bible expresses 1260 days as 3.5 times (also as 42 months in Rev 11). 1260/3.5 = 360. We have to go with what was the case when the verses were penned, not what was centuries established later.
Even the 70 years has a drift. 70 lunar years equates to 69.04 years (using 365 days) or 68.99 years (using 365.25 days, and even that's not entirely accurate.)
Regardless, this prophecy is completely made up to begin with. Connecting Jesus words to Daniel 4 just is mind-boggling. And the prophecy didn't originate with JWs. It goes back to a Baptist minister named William Miller, if I recall correctly, then through the Adventists. JWs just tweaked it to match up with 1914 somehow.
Back in 1934, in the 3/28 "Golden Age", they did indeed claim to use lunar years of 360 days. The problem is that they use lunar years to get 2,520, but then count forward using solar years.