Long before man’s creation, God provided the basis for such measuring of time. Genesis 1:14, 15 tells us that one of the purposes of the “luminaries in the expanse of the heavens” is that they might serve for “seasons and for days and years.” The solar day, the solar year, and the lunar month are thus natural divisions of time, governed respectively by the daily turning of the earth on its axis, by its annual orbit around the sun, and by the monthly phases of the moon in its relation to earth and sun.

Monthly divisions also came into use. By the time of the Flood we find time divided into months of 30 days, since a period of 5 months is shown to equal 150 days. (Ge 7:11, 24; 8:3, 4) The same record also indicates that Noah divided the year into 12 months.​—See YEAR.

Early calendars were mainly lunar calendars, that is, the months of the year were counted by complete cycles of the moon, as, for example, from one new moon to the next new moon. On the average, such lunation takes about 29 days, 12 hours, and 44 minutes. The months were usually counted as of either 29 or 30 days, but in the Bible record the term “month” generally means 30 days.​—Compare De 21:13; 34:8; also Re 11:2, 3.

A year of 12 lunar months falls about 11 days short of a solar year of 365 1⁄4 days. Since the solar year determines the return of the seasons, there was need to adjust the calendar to this solar year, and this resulted in what are called lunisolar, or bound solar, years​—that is, years in which the months were lunar but the years were solar. This was done by the addition of a number of days each year or of an additional month during certain years to compensate for the shortness of the 12 lunar months.

The Bible does not indicate what method was originally used to determine when additional days or an additional, or intercalary, month should be inserted. It is logical, however, that either the vernal or the autumnal equinox served as a guide to indicate when the seasons were falling behind sufficiently to require calendar adjustment. Though not specifically mentioned in the Bible, a 13th month that was added by the Israelites to accomplish this adjustment was called, in postexilic times, Veadar, or the second Adar.

We do not find record of a definitely fixed or standardized form of Jewish calendar until the fourth century of our Common Era (c. 359 C.E.), when Hillel II specified that the leap years of 13 months should be the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th of each 19 years. Such a 19-year cycle is commonly called the Metonic cycle, after the Greek mathematician Meton (of the fifth century B.C.E.), although there is also evidence that such a cycle was perfected before him by the Babylonians. (See Babylonian Chronology, 626 B.C.–A.D. 75, by R. A. Parker and W. H. Dubberstein, 1971, pp. 1, 3, 6.) This cycle takes into account that every 19 years the new and the full moons fall again on the same days of the solar year.

The Jewish months ran from new moon to new moon. (Isa 66:23) Thus, one Hebrew word, choʹdhesh, rendered “month” (Ge 7:11) or “new moon” (1Sa 20:27), is related to cha·dhashʹ, meaning “new.” Another word for month, yeʹrach, is rendered “lunar month.” (1Ki 6:38) In later periods, fire signals were used or messengers were dispatched to advise the people of the beginning of the new month.

In the Bible the individual months are usually designated simply by numbering according to their position in the year, from the 1st through to the 12th. (Jos 4:19; Nu 9:11; 2Ch 15:10; Jer 52:6; Nu 33:38; Eze 8:1; Le 16:29; 1Ki 12:32; Ezr 10:9; 2Ki 25:1; De 1:3; Jer 52:31) Only four months are named prior to the exile in Babylon, namely, Abib, the first month (Ex 13:4); Ziv, the second (1Ki 6:37); Ethanim, the seventh (1Ki 8:2); and Bul, the eighth (1Ki 6:38). The meanings of these names are strictly seasonal, thus giving additional proof of a lunisolar year.​—See the individual months by name.

In postexilic times the names of the months used in Babylon were employed by the Israelites, and seven of these are mentioned: Nisan, the 1st month, replacing Abib (Es 3:7); Sivan, the 3rd month (Es 8:9); Elul, the 6th (Ne 6:15); Chislev, the 9th (Zec 7:1); Tebeth, the 10th (Es 2:16); Shebat, the 11th (Zec 1:7); and Adar, the 12th (Ezr 6:15).

The postexilic names of the remaining five months appear in the Jewish Talmud and other works. They are Iyyar, the 2nd month; Tammuz, the 4th; Ab, the 5th; Tishri, the 7th; and Heshvan, the 8th. The 13th month, which was intercalated periodically, was named Veadar, or the second Adar.

Eventually the length of most of the months was fixed as having a specific number of days. Nisan (Abib), Sivan, Ab, Tishri (Ethanim), and Shebat regularly had 30 days each; Iyyar (Ziv), Tammuz, Elul, and Tebeth regularly had 29 days each. Heshvan (Bul), Chislev, and Adar, however, could have either 29 or 30 days. The variations in these latter months served to make necessary adjustments with the lunar calendar but also were used to prevent certain festivals from occurring on days viewed as prohibited by later Jewish religious leaders.