A division of the year derived from the period required by the moon to pass through its 4 phases, as from one new moon to the next. Since the time of the moon’s circuit around the earth is neither an exact number of days nor a commensurable part of the year—its synodical revolution, approximately 29 1/2 days—a calendar month must be either a variable or an artificial period.
The Hebrew month was lunar, beginning with the evening on which the crescent moon appeared. The 1st day of the month was called the new moon (see 1 Sa 20:24–27), and was a day of special observances (see Num 10:10; 1 Sa 28:11–15). At first, visual observation was used to determine the appearance of the crescent. If the crescent was seen on the evening following the 29th day of the month, a new month had begun; if not, another day was added so that that particular month had 30 days (a month never had more than 30 days). Later, methods of calculation were devised by which it could be known in advance whether a given month should have 29 or 30 days. Jewish tradition tells of the method of examining at Jerusalem witnesses who claimed to have seen the crescent, to determine whether they had actually seen it or not. It tells also of fire signals that were used to announce the beginning of the new month to the outlying areas.
The Babylonians, like the Greeks, had lunar months. The Egyptian months were 30 days each, with 5 extra days after the 12th month. The Roman months, originally lunar, were changed to the 30, 31, and 28 (29) days that we know today. Modern Jews still use their Biblical months for religious purposes. These months generally alternate between 30 and 29 days, which have been calculated for centuries according to variable but standardized rules, and are no longer dependent on the moon. The Moslems, on the other hand, still depend on the actual sighting of the crescent moon each month for their religious calendar.
Neither the 30-day month implied in the Flood narrative (150 days totaling 5 months; Gen 7:11, 24) nor the 30-day prophetic month (42 months equaling 1260 days; Rev 11:2, 3; see Time, 5), has anything to do with the Jewish calendar month. Genesis does not furnish enough information to warrant drawing conclusions as to the sort of calendar Noah might have employed, and the 30-day month of Revelation is prophetic and symbolic, not literal, for no known calendar runs in an unbroken series of 42 30-day months. However, the idea of a theoretical or ideal month of 30 days was logical to the Jews, who called 30 days a “full” month; a 29-day month they spoke of as “hollow,” or “deficient.” Because of the variability of the moon’s motion, lunar months do not invariably alternate 29 and 30 days. Consequently it was impossible for the ancient Jews and Babylonians to predict the exact number of days in a period of months or years ahead. Thus the logical method of reckoning a future period was to count by “full” months, that is, theoretical months, of 30 days each (even as we today sometimes count 30 days to the month in computing interest). The Babylonians did this and quite possibly the Jews did also. Since not even Jewish readers of the book of Revelation could tell exactly how many days were in 42 Jewish months without knowing which months they might be (nor could the Gentiles using the Roman calendar, without taking leap years into account), the most logical method of reckoning a prophecy expressed in months and relating to future time would be by theoretical, rather than calendar, months. And the fact that the 42 months are of 30 days each is clear from the equation with 1,260 days. Further, months in symbolic prophecy are not literal, but symbolic months (which, interpreted by the year-day principle, are each 30 years) ( pp. 757, 758)