The Bible uses different calendars at different stages of God's Plan for the Ages.

Types of calendar

• LUNAR - months start at a new moon and the year has 12 lunar months. The calendar drifts from the solar seasons by 11 days each year.
• LUNISOLAR - months still start at a new moon and the year normally has 12 months, but an extra month is added every two or three years to keep in line with the solar seasons.
• SOLAR - the year has 365 days extended by a day in leap years to keep in line with the solar seasons. Month lengths vary and so months are unlikely to start at a new moon.

Lunar

The time from new moon to new moon is called a lunation, and is a month as we most commonly understand it. It is the mean time for one orbit of the moon round the earth, returning to the same place in the sky relative to the sun. Its value today ranges from approximately 29.27days to 29.84 days, with an average of 29.530589 days.

Before the Flood the Biblical year had 12 months. It's not possible to organise a calendar with 29.5 days in a month. We've chosen to make odd months 30 days long and even months 29 days long, producing a twelve-month year of 354 days. A lunar year comprising 12 months of 354 days is approximately 11 days shorter than a solar year of 365¼ days.

Before the Flood it seems as though there may have been a canopy of water vapour over the earth that produced a "greenhouse" effect. There would have been no agricultural seasons linked to the sun.

Lunisolar

The water canopy collapsed at the Flood and the "greenhouse" effect would have come to an end. So, after the Flood agriculture became governed by the solar seasons. This meant that the calendar had to change to keep in line with the solar seasons and an extra (intercalary) month had to be introduced every 2 or 3 years as needed to keep years in line with the sun. This intercalated lunar calendar is called a lunisolar calendar. It is complicated because the calendar has to harmonise simultaneously with both lunar and solar events.

Then, after the Exodus God introduced feasts linked to the solar agricultural seasons to Israel. This meant that Israel had to continue with a lunisolar calendar so that the feasts occurred at approximately the same time each year. Otherwise the feast would advance by 11 days a year and the harvest feasts of the first 3 months of the year (roughly April - June) would end up in the winter within a few years.

The modern Jewish Calendar is worked out on this basis and does not account for a 12 month lunar year before the Exodus.

Until the 4th century AD, the start of the Jewish Calendar year was determined by observation of the phasis of the moon, that is the first appearance of the crescent moon which would not be visible until at least a day after the true astronomical new moon, depending on the timing of that new moon and the weather conditions.

Solar

These calendars are linked to the solar year of 365.242199 days.

Summary of Calendars

354-day Creation Calendar

This is the lunar based 354-day calendar that is in operation during creation week. We only display it during that time and for Adam's life which of course begins during creation week!

354-day Gentile Calendar

This is the lunar based 354-day calendar that the Bible uses after creation week until the Flood, so it's always seven days behind the 354-day Creation Calendar.

360-day Judgement Calendar

This is the 360-day calendar that operates during the two periods of judgement on the earth, the Flood and the Tribulation that is yet to come.

Lunisolar Autumn Calendar

This is the lunisolar calendar in operation from the Flood to the Exodus.

Lunisolar Spring Calendar

This is the lunisolar calendar from the Exodus until the death of Jesus. It starts in spring rather than the autumn and so is six months apart from the previous calendar. It overlaps with the True Solar Calendar of the life of Jesus.

True Solar Jesus Calendar

This is the True Solar Calendar of the life of Jesus.

Julian Solar Calendar

The Julian Calendar was introduced on 1 January 45 BC by Julius Caesar, and is the calendar that was in use by the Romans during the New Testament period.

Gregorian Solar Calendar

This is the calendar in use today in most countries. It differs from the Julian Calendar in its treatment of leap years and Easter. The date of Easter had been laid down at the Council of Nicea in 325 AD. By the 16th century, Easter was occurring later in relation to the spring equinox. The idea of the Gregorian Calendar was to bring Easter back to where it would have been if the Julian Calendar in use since the Council of Nicea had been astronomically correct.

Astronomical Data

NASA publish astronomical data going back 4000 years in the public domain, and the algorithms used by astronomers to work out the times of new moons and equinoxes are freely available today.

However, we have made some important adjustments.

Sun stands still for 10 hours during the reign of Hezekiah

Following the defeat of Shalmaneser (2Kings 19:35-36), 2Kings 20:1-11 and Isaiah 38:1-8 tell the story of the sickness of Hezekiah when he was near to death. Isaiah told those attending Hezekiah to apply a lump of figs as a poultice to the boil and prophesied that on the 3rd day Hezekiah would be well enough to attend the Temple and would live for another 15 years. Hezekiah asked for a sign that God would do what Isaiah had prophesied, so Isaiah again prophesied that the sun would return 10 degrees or steps representing 10 hours on the sundial as the sign. This happened, Hezekiah was healed and lived for another 15 years.

The Talmud relates discussion in the Sanhedrin that shows that the Rabbis understood these degrees to be Jewish hours. See NOTES for the events Jesus born and Death of Jesus.

Although the sun stood still for 10 hours, there's no mention in the Bible that the moon did! Therefore astronomical calculations for new moons add 10 hours to adjust for this BEFORE this occurrence.

Location of Jerusalem

Astronomical data generally uses GMT - Greenwich Mean Time. However, the start of lunisolar months in the Bible was determined by observation of the crescent new moon in the Middle East, so we have adjusted new moons by 2 hours 20 minutes to allow for the time difference.

Delay before the new moon was observed

We have allowed 30 hours for the delay before the crescent new moon would be visible in the Middle East.