With the ancient Israelites the first day of a new moon was the first day of a new month, so that a month would consist of either 29 or 30 days. In the Old Testament the months are not usually named but simply numbered; they spoke of “the second month” and so on. The later Jews called their months by names they got from Babylon: (1) Nisan, (2) Iyar, (3) Siwan, (4) Tammuz, (5) Ab, (6) Elul, (7) Tishri, (8) Markheshwan, (9) Kislew, (10) Tebeth, (11) Shebat, (12) Adar. To these was added, when necessary, a thirteenth month, called 2nd Adar. With the later Jews the civil year began on the 1st of Tishri. Besides these Babylonian names we find in the Old Testament four names of months that were in use among the Jews before the Exile: Abib (“harvest month,” Ex. 13:4; 23:15; 34:18; Deut. 16:1), which corresponded to the later Nisan; and Ziw (or Zif, 1 Kgs. 6:1, 37); Ethanim (1 Kgs. 8:2); and Bul (1 Kgs. 6:38), which are stated to be the second, seventh, and eighth month respectively.
The day among the Hebrews was reckoned from sunset to sunset (Lev. 23:32). In the Old Testament no divisions of the day are mentioned except the natural periods of morning, noon, and evening. The night was divided into three watches (Judg. 7:19). In later times the number of night-watches was increased to four (Matt. 14:25; Mark 6:48) in accordance with Greek and Roman custom. The hours of the day were usually counted from sunrise, the hours of the night from sunset.