Boyd’s Bible Dictionary:
The Hebrew year was sacred and civil, with two beginnings
The sacred year began with the month Abib, April, the civil with the month Tisri, October. The months were lunar, twelve in number, with, of course, the necessary intercalary month ve-adar at the proper time, about every three years. As divided by seasons, the year was solar. There were two seasons, summer and winter (Psa. 74:17; Jer. 36:22; Amos 3:15; Zech. 14:8).
Concise Bible Dictionary:
Under the word MONTHS it has been stated that the Jews reckoned the months to consist alternately of twenty-nine and thirty days, being therefore in twelve months eleven and a quarter days short of the year. To remedy this an additional month was added about every three years. In the various data given for the last half of the last of Daniel's Seventy Weeks, it will be seen that all the months are reckoned as having thirty days; thus “a time, times, and a half “ in Daniel 12:7 and Revelation 12:14 point out three and a half years: this period is again called forty two months in Revelation 11:2 and Revelation 13:5; and again twelve hundred and sixty days (Rev. 11:3; Rev. 12:6). The prophetic year may therefore be called three hundred and sixty days. See MONTHS and SEASONS.