Among the Egyptians the month of thirty days each was in use
long before the time of the Exodus, and formed the basis of
their calculations. From the time of the institution of the
Mosaic law the month among the Jews was lunar. The cycle of
religious feasts depended on the moon. The commencement of a
month was determined by the observation of the new moon. The
number of months in the year was usually twelve (1 Kings 4:7; 1
Chr. 27:1-15); but every third year an additional month
(ve-Adar) was inserted, so as to make the months coincide with
the seasons.

"The Hebrews and Phoenicians had no word for month save
'moon,' and only saved their calendar from becoming vague like
that of the Moslems by the interpolation of an additional month.
There is no evidence at all that they ever used a true solar
year such as the Egyptians possessed. The latter had twelve
months of thirty days and five epagomenac or odd days.",
Israel Quarterly, January 1889.