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. The division of time into weeks and the division of the day into hours, on the other hand, are arbitrary ones.

From the first man Adam forward, time has been measured in terms of years. Thus, Adam was “a hundred and thirty years” of age when he became father to Seth.​—Ge 5:3.

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.

Hebrew Calendar. The Israelites used such a lunisolar, or bound solar, calendar. This is evident from the fact that Jehovah God established the beginning of their sacred year with the month Abib in the spring and specified the celebration of certain festivals on fixed dates, festivals that were related to harvest seasons. For these dates to have coincided with the particular harvests, there had to be a calendar arrangement that would synchronize with the seasons by compensating for the difference between the lunar and solar years.​—Ex 12:1-14; 23:15, 16; Le 23:4-16.

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.

Whereas the sacred year began in the spring with the month Abib (or Nisan) by God’s decree at the time of the Exodus (Ex 12:2; 13:4), the Bible record indicates that prior to this the Israelites had counted the year as running from fall to fall. God gave recognition to this arrangement so that, in effect, there was a dual system of a sacred and a secular or agricultural calendar used by his people. (Ex 23:16; 34:22; Le 23:34; De 16:13) In postexilic times, Tishri 1, in the last half of the year, marked the beginning of the secular year, and the Jewish New Year, or Rosh Hashanah (head of the year), is still celebrated on that date.

In 1908 the only approximation of an ancient written Hebrew calendar was found at the site of Gezer, and it is believed to be from the tenth century B.C.E. It is an agricultural calendar and describes agricultural activity beginning with the autumn. In brief, it describes two months each of storage, sowing, and spring growth, followed by one month each of pulling up flax, barley harvest, and a general harvest, then two months of pruning the vines and, finally, one month of summer fruit.​—Le 26:5.

The chart accompanying this article shows the months in their relation to both the sacred and secular calendars and also their approximate correspondence to the months of our present calendar.

Julian and Gregorian Calendars. In the year 46 B.C.E., Julius Caesar issued a decree changing the Roman calendar from a lunar to a solar year. This Julian calendar, based on the calculations of the Greek astronomer Sosigenes, had 12 months of arbitrary length and a regular year of 365 days beginning on January 1. It also brought in the use of leap years by the addition of an extra day every four years, to compensate for the extra fraction of a day in the length of the tropical year, which has a little less than 365 1⁄4 days.

The Julian calendar year was actually a little more than 11 minutes and 14 seconds longer than the true solar year. Thus, by the 16th century a discrepancy of ten full days had accumulated. In 1582 C.E., Pope Gregory XIII introduced a slight revision of the Julian calendar, whereby the leap years every four years were retained but with the exception that only those century years with a number divisible by 400 were to be counted as leap years. By papal bull in 1582, ten days were to be omitted in that year, so that the day after October 4 became October 15. The Gregorian calendar is now in general use in most parts of the world. It is the basis for the historical dates used throughout this publication.