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The length of a solar year is close to 365.1/4 days
The length of a lunar month is close to 29.1/2 days.
The solar and lunar cycles realign every 19 years. In Biblical times, the arrival of the new month was determined by watching the phase of the moon. 12 lunar months make about 354 days, which is 11.1/4 days shorter than the solar year.
The difference of 11.1/4 days between 12 lunar months and one solar year accumulates in three years to more than a month. If no adjustments are made, summer months would shift to the winter.
Because the Jewish holy days are closely related to the seasons (for example, Passover is to be celebrated in the spring and the sheaf of firstfruits waved on the day following the Passover Sabbath; see Leviticus 23), an adjustment to the calendar must be made every few years.
Today in the Jewish Calendar, every two or three years, one extra month is added to a year; such a year is known as a leap year and has two months of Adar called, unsurprisingly enough, Adar1 and Adar2; but it is not certain which month was repeated in Old Testament times. The Jews have a 'civil year' which begins in the Autumn and a 'religious year' which begins in the spring.
Because days cannot be 'half' days and each month began with the appearance of the 'new' moon, it follows that lunar months can have either 29 or 30 days. The situation is further complicated by the Jewish day beginning in the evening. A Biblical year then can vary between 12 and 13 lunar months.
For prophetic time periods a month is generally assumed to have 30 days and year to have 360 days. In this way 'time, times and a half' (Daniel 12:7) = 3.1/2 years = 1260 days = 42 months; which, if a day is taken to represent a year, becomes 1260 years.
In conclusion, it is impossible to be precise about the Old Testament Biblical year, because although we know that months began with the new moon and adjustments are necessary to keep the seasons in place we have insufficient information to know exactly how Israel did it.
I hope this helps.