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Kitto's Popular Cyclopedia of Biblical Literature
The Hebrew year consisted of twelve unequal months, which, previously to the exile, were lunar. The twelve solar months made up only 354 days, constituting a year too short by no fewer than eleven days. This deficiency would have soon inverted the year, and could not have existed even for a short period of time without occasioning derangements and serious inconvenience to the Hebrews, whose year was so full of festivals. At an early day, then, we may well believe a remedy was provided for this evil. The course which the ancients pursued is unknown, but Ideler (Chronol. i. 490) may be consulted for an ingenious conjecture on the subject. The later Jews intercalated a month every two or every three years, taking care, however, to avoid making the seventh an intercalated year. The supplementary month was added at the termination of the sacred year, the twelfth month (February and March), and as this bore the name of Adar, so the interposed month was called Veadar, or Adar the Second. The year, as appears from the ordinary reckoning of the months (; ; ; ; ; comp. ; ), began with the month Nisan (), agreeably to an express direction given by Moses (; ). This commencement is generally thought to be that of merely the ecclesiastical year; and most Jewish, and many Christian authorities, hold that the civil year originally began, as now, with the month Tisri. The ancient Hebrews possessed no such thing as a formal and recognized era. Their year and their months were determined and regulated, not by any systematic rules of astronomy, but by the first view or appearance of the moon. In a similar manner they dated from great national events, as the departure from Egypt (; ; ); from the ascension of monarchs, as in the books of Kings and Chronicles; or from the erection of Solomon's temple (; ); and at a later period, from the commencement of the Babylonish captivity (; ). When they became subjects of the Graeco-Syrian Empire they adopted the Seleucid era, which began with the year B.C. 312, when Seleucus conquered Babylon.
Kitto, John, ed. Entry for 'Year'. "Kitto's Popular Cyclopedia of Biblical Literature". https://www.studylight.org/encyclopedias/eng/kbe/y/year.html.