As I understand it each year has 12 or 13 months. The Jewish calender today with error I believe predicts on a 19 year cycle if there is to be a 13th month.
Biblically the first month is determined (or the 13th month added) by observing the barley crop which has to be at a certain stage (for the firstfruits offering) to call the first month of the year. It is the only month of the year with a GOD given name "Abib" or "Aviv" we call it Nisan which is the Babylonian name.
In short there is no biblical way to predict a year it swings one way or another by 29 days to be determined each spring.
There...Ive said it.