The Biblical calendar is the calendar most likely used in Biblical times, certainly by the ancient Hebrews, and possibly by antediluvian Sethites as well. Noah undoubtedly used that calendar to record key dates during the Flood—or God revealed those dates directly to Moses.
History and Reconstruction
James Ussher assumed that the ancient Hebrews and Israelites used a variant of the calendar that the ancient Egyptians used. This was a solar calendar consisting of twelve months of thirty days each, with five (or, every four years, six) days added at the end. The start of this calendar was, supposedly, the Date of Creation.
|“||It cannot be proven that the Hebrews used lunar months before the Babylonian captivity.||”|
|“|| The Ancient year of the Latines was also Luni-solar; for Plutarch  tells us, that the year of Numa consisted of twelve Lunar months, with intercalary months to make up what the twelve Lunar months wanted of the Solar year. The Ancient year of the Egyptians was also Luni-solar, and continued to be so 'till the days of Hyperion, or Osiris, a King of Egypt, the father of Helius and Selene, or Orus and Bubaste: For the Israelites brought this year out of Egypt; and Diodorus tells  us that Ouranus the father of Hyperion used this year, and  that in the Temple of Osiris the Priests appointed thereunto filled 360 Milk Bowls every day: I think he means one Bowl every day, in all 360, to count the number of days in the Calendar year, and thereby to find out the difference between this and the true Solar year: for the year of 360 days was the year, to the end of which they added five days.
That the Israelites used the Luni-solar year is beyond question. Their months began with their new Moons. Their first month was called Abib, from the earing of Corn in that month. Their Passover was kept upon the fourteenth day of the first month, the Moon being then in the full: and if the Corn was not then ripe enough for offering the first Fruits, the Festival was put off, by adding an intercalary month to the end of the year; and the harvest was got in before the Pentecost, and the other Fruits gathered before the Feast of the seventh month.
 Plutarch. in Numa.
 Diodor. l. 3. p. 133.
 Diodor. l. 1. p. 13.
The references from Diodorus Siculus almost certainly post-date the Exodus of Israel. In summary, Newton confirms Ussher's deduction that the Israelites used a variant of the Egyptian calendar, but rejects Ussher's apparent presumption that that calendar was a solar calendar with strict thirty-day months.
Jones accepts Sir Isaac's statements about the Hebrew calendar and offers a detailed explanation. The Hebrew calendar has always been luni-solar in nature. Hillel II established his calculated luni-solar calendar with the nineteen-year cycle on or about 350 AD.
Earlier than that, and beginning with the destruction of the Temple of Jerusalem by Nebuchadnezzar II, the Hebrews used an eight-year cycle, with an intercalary Adar I between Shevat and Adar II(VeAdar) inserted on the 3rd, 6th, and 8th year of the cycle. All the other months alternated without fail between 30 and 29 days, beginning with Abib (Nisan) (30) and Zif (Iyar) (29).
But the original Biblical calendar was not calculated at all, but truly observed. That is to say, it depended strictly on two things: the position of the moon and the ripening of the barley crop. It likely began with the month Eitamin (Tishrei) beginning with the new moon that immediately followed the day of creation.
|Abib (barley, head)||Exodus 23:15||Nisan||Barley harvest, lentils||Latter rains|| 14 - Passover|
15-21 - Unleavened bread
17 - Firstfruits
|Zif||I_Kings 6:1||Iyar, Iyyar||General harvest||Latter rains||None|
|"Wheat harvest"||Esther 8:9||Sivan||Wheat harvest||Dry||7 - Feast of Weeks (Pentecost)|
|(Not found)||(Not attested)||Tammuz||Vine tending||Dry||None|
|(Not found)||(Not attested)||Av||First grapes, figs, olives||Dry||None|
|Elul||Nehemiah 6:15||Elul||Grapes, dates||Dry||None|
|Ethanim or Tishrei||I_Kings 8:2||Tishrei||Vintage, plowing||Dry to early rains|| 1 - Feast of Trumpets|
10 - Atonement
15-21 - Feast of Tabernacles
|Bul||I_Kings 6:38||Marchesvan, Cheshvan||Wheat, barley, seed sowing||Early rains||None|
|Chisleu or Kislev||Zechariah 7:1||Kislev||None||Cool, rain||25 - Feast of Dedication begins|
|Teveth||Esther 2:16||(same)||None||Cold, rain||1-2 or 1-3 - Feast of Dedication ends|
|Shevat||Zechariah 1:7||(same)||Winter figs, citrus harvest||Rain||None|
|Adar (Iron, strength)|| Esther 3:7
|(same)||Almonds blossom, flax harvest||Rain|| 13 - Fast of Esther|
14-15 - Purim
A month in the Biblical calendar ends on the day that the new moon is sighted at sunset, and the next month begins on the next day. Given the length of the synodic month, a Biblical month could run either 29 or 30 days. When weather conditions make a new-moon sighting problematic, the ranking authority (presumably the patriarch of the family, and later a council appointed among the Levites) would declare that the month would last for thirty days, and no longer (except possibly for Adar; see below). The first reliably sighted new moon would begin another month, and so this calendar could correct itself automatically from any missed cycles.
The first month of spring, called Abib or Aviv, is a special case. It begins on a new moon, but only when the barley has turned green and will likely ripen before the Feast of Firstfruits, which must take place on the Sunday (actually called simply "first day (of the week)") following Passover. If this has not happened, then the new moon is not declared and Adar runs longer than the usual 29 or 30 days.
The best approximation from astronomy would be to begin Abib on the new moon nearest the vernal equinox. At least one authority suggests adding five to six days to make sure that day and night will be equal in length in the latitude of Jerusalem, not at the equator, but this is not proved. No such month as VeAdar is named in the Bible, nor is its use proved or suspected until after the captivity begins.
The epoch of this calendar is likely the new moon that would have followed Creation, had the moon existed at the time. In that year, Abib must have begun no sooner than six lunar cycles after Ethanim (to account for the one-month-late beginning of the year after Creation).
The very notion of using the vernal equinox to predict the beginning of Abib, and hence Passover, is in sharp dispute. Many authorities hold that the only predictor of Abib in ancient times was the status of the barley crop in the Land of Israel. Others hold that Abib ought to begin on the new moon immediately following the vernal equinox, without fail. At least one commentator holds that fixing Abib to begin on the new moon nearest the equinox is the best estimator of when the barley is likely to come to a green head in Israel.
Symbolism and specific Bible history
The month Ethanim (Tishrei) was originally called "the first month," and all the other months were numbered from that month, as one can see in the account of the Global flood. This rule changed in Exodus 23:15 , wherein God commanded the Israelites to observe Abib, not Ethanim, as the first month.
Furthermore, the selection of the almond as the basis of the seven-member lampstand(Exodus 25:31-40 , Exodus 37:17-34 ) is telling. When the almonds bloomed, the Israelites used that as a sign to watch the barley crop for the first signs of ripening. This rule was the same even during Sabbath or Jubilee years, and was observable because barley will grow even if uncultivated, and probably grew in sufficient profusion for the Israelites to "harvest" it even when they had not planted it.
The Babylonian Captivity (586 BC) severed the tie that the Israelites (or to be more specific, the Jews) had to the land. Living in exile in Babylonia, they came under the influence of Babylonia's astronomers and learned their ways. At this time they developed a calculated calendar, with alternating 30 and 29-day months, and intercalated 30-day months every third, sixth, and eighth year. This solution served them well until about 350 AD, when they must have realized that the calendar had begun to drift. Hillel II devised the nineteen-year cycle that the Jews use today.
For political reasons, the Roman Catholic Church established the tradition of observing Resurrection Sunday, or Easter, on the Sunday following the first full moon that falls on or after the vernal equinox. By coincidence, this would place it exactly on the Sunday following Passover according to this calendar, and would be in keeping with Jesus Christ offering Himself as a "firstfruit" for all of humanity. But this is not always the case, and Catholic history clearly shows that the Catholic church desired to calculate Resurrection Sunday independently of any calendar that the Jews used.
Speculation on what sort of calendar Jesus Christ will decree when He returns to earth to rule it directly is probably vain. Nevertheless this Biblical calendar is the best candidate for that honor, primarily on account of the detailed and strict instructions that God gave for its observance.
- Jones, Floyd N. The Chronology of the Old Testament, 16th ed. Green Forest, AR: Master Books, 1993-2007 (ISBN 9780890514160), pp. 106-109
- James Ussher, The Annals of the World, Larry Pierce, ed., Green Forest, AR: Master Books, 2003 (ISBN 0890513600), p. 9
- Newton, Sir Isaac. The Chronology of Ancient Kingdoms Amended. Reproduced by Project Gutenberg. Accessed April 3, 2008.
- Abib, Zif, Ethanim, and Bul are the Hebrew names. Nisan, Iyar, Tishrei, and Marcheshvan (abridged to Cheshvan today) are the Aramaic names.
- Jones, op. cit., Appendix E, p. 277
- Nehemiah 2:1
- Also known as Feast of Lights, or Chanukkah.
- Abib and Aviv are the same word. In Semitic languages, "b" and "v" are the same sound; the distinction between the plosive "b" and the fricative "v" is of no moment.
- Tippie, Robert. "The Barley is Ripe in Israel: Implications for the Dating of the Passover Celebration." B'nai Avraham Nazarine Yisroelite Synagogue. Accessed April 3, 2008
- "The Calendar: Equinox or Barley?" Yahweh Restoration Ministries. Accessed April 3, 2008.
- "Abib FAQ." Karaite Korner, December 8, 2003. Accessed April 3, 2008.
- "Choose Your Barley, Part 2: The Barley/Equinox Question." Truth of Yahweh. Accessed April 3, 2008.
- Arnow, David. "Passover and Barley?" <http://livelyseders.com>. Accessed April 3, 2008. This reference also gives directions for planting an indoor barley crop to provide barley for Passover Seder.
- This is why the Historical date datatype, developed specially for CreationWiki, uses this method to fix the beginning of Abib in the AM implementation.
- Revelation 19-20