Actually is right with a wrong explanation, let me explain, then feel free to comment.
Jewish calendar is not lunar but It is both lunar and solar as Gen 1:14 states :
Then God said: “Let there be luminaries*+ in the expanse of the heavens to make a division between the day and the night,+ and they will serve as signs for seasons and for days and years.
They(jews) counted 360 days a year but adding the extra month as explained in blondie's comment above.
Please take note in this verse that the sun and the moon serve not only as to determine the days, and years, but also seasons and signs.
This prophecy (the 2520 years) is highly important, you can accept or refuse its explanations.
Please consider too the "day of Jehovah", which is related also to "time" and "seasons" 2 Thess 5: 1 3.
Now as for the times and the seasons, brothers, you need nothing to be written to you.
2 For you yourselves know very well that Jehovah’s* day+ is coming exactly as a thief in the night.+
3 Whenever it is that they are saying, “Peace and security!” then sudden destruction is to be instantly on them,+ just like birth pains on a pregnant woman, and they will by no means escape.
If you are sincere with bible text consider attentively each word, as highly important.
Nobody knows the date or the hour, but from revelation verse we know it won't surprise the anointed.
Don't serve men, or for a date, but Jehovah for He deserve the adoration as our Grand Creator.
Last thing, Jw are not inspired but guided, meaning they made mistakes, and still do, so be sincere, and go back always to check what Bible tells. In my humble opinion they tell the truth, they could be wrong and fell 7 times, we need to forgive them.... we are all humans, we all do mistakes.
Aleph