From 607 BCE I understand how the 2520 years can add up to 1914, but I don't understand the arguments against the WT years calculation.
The WT say there are 2520 years from 607 BCE (forget that this isn't a correct date to start with).
It doesn't mention months or gregorian or lunar or anything, just "years".
If I were to ask a Jewish person "what year will it be in 15 years time?" they would look at their calendar, do their sums, and tell us the year. They wouldn't convert to lunar, misplace the leap-months, start adding days etc, they would just add 15 Jewish years to get a result.
The 2520 years is the same.
To my mind this is a non-argument that any JW can demolish just by adding 2520 years to 607 BCE.
The Jewish new year, Rosh Hashanah, is always in autumn. This is in the month of Tishri and corresponds typically to some time in September but occasionally October. It was instituted as a celebration in Lev 23:24-25.
Add 2520 autumns (Jewish years) to any Gregorian date, and you get the date 2520 years later.
To conflate this with all the lunar, leap, Jewish, Gregorian stuff is exactly what WT do to prove a point.
The fatal error in WT calculations is 607 BCE, not the 2520 year calculation.