The Bible was written using a specific calendar. That calendar is not used anywhere in the world today. Its structure defines how God counts days, months, years and larger quantities of time. When God says 1000 years are as a day, he means 1000 years on this calendar..
Taken together, the references to time given in the bible do not allow for any modern calendar to be the calendar used when the Bible was written. The following calendar is adequate for describing all dates given in the Bible. This is the calendar used in this site for explaining the biblical chronology.
There is scripture for defending both sunrise and sunset as the time when the day should change. Such statements as God’s mercies are "new every morning" suggest new every day. The seven days of creation are written with morning as their point of change. Jesus is the "Morning Star" meaning he is the light of a new day. When we choose to follow him we are starting a new day and he is the light of that day. This too suggests that the day-break point for God is morning.
There are also references that suggest sunset as the day change point. Jesus was in the tomb three days, Thursday afternoon through Sunday morning. If the day-break point is at sunrise then sunrise on Saturday is the only daybreak in this period, breaking this time interval down into 2 days, not three.
So when does the day change? At what we call Sunset, or on a mechanical approximation of sunset.
The Bible does not provide a reference location for observing or computing sunset. But, Jerusalem is the city that God choose to put his name, to house the ark, and to set up the Temple. This city is the natural location for computing time world wide.
Times earlier than the establishment of Jerusalem as the key city are considered to be acceptable if they are base on computations back from the time system at Jerusalem.
This makes sense since the Bible does not contain any record of major time corrections. (Except for the sun standing still in Joshua’s time and Hezekiah’s time.)
The worldwide nature of the Bible’s clock suggests prophetic events happen as timed in various places around the world. Correction back to Jerusalem time is considered the best strategy for measuring prophetic time.
The Bible Calendar, which could also be called the Temple Calendar consists of a month who’s length is always 30 days. Never 28, 29 or 31 days. It is constructed atop the definition of day given above. Months are considered a perfect form of measurement.
Though considered perfect, care must be taken when the tolerance issues of biblical passages are factored in. If God implies something will take place within a given month, the month is defined perfectly but the day within the month remains unknown. Any date within that month is within the tolerance of a month.
The biblical year consists of both common and leap years. The common biblical year is made up of twelve 30 day months for a total of 360 days. Leap years are made up of 13 months of 30 days each, for a total of 390 days. Every seventh year through the 49th year is a leap year as well as the 50th year. After the 50th year the cycle begins again. This makes each half century identical in structure to every other half century.
There are 18240 days in each half century. The average number of days in a biblical year is 18240 / 50 = 364.8 days.
The Biblical calendar has various starting epochs. As written the Bible restarts epochs with each new patriarch or king. This is what leads to the uncertainty of the overall span of time recorded in the Bible since epoch overlap is not directly addressed.
Two special epochs deserve special mention here. The first begins with the first year of Adam’s life. All years counted from this point are known as years "After Adam" or AA for short. The second epoch is the first year after the Exodus. These year numbers are counted "From Exodus" or FE for short. The two systems are exactly 9500 years apart, so the Sabbath and Jubilee system is not interrupted by even one day.
Other Epochs can be displayed using the tool. Simply select the name of the person in question and the adjustment will be made for that epoch.
Day counting systems
There are many places in the bible where very big day counts are given. Doing math on year/month/day tuples does not lend itself well to computations across very large day ranges. To aid day based arithmetic all dates on this calendar are assigned an "After Adam Day Number", or AA for short. These are always presented by the database query programs. Use the AA tools page to look up a date or series of dates using strictly these numbers. The first day of the first month of Adam is 1/1/1 AA. We are more than 4.7 million such days past Adam.
Negative day numbers are possible, there is a day zero so arithmetic operations on days work across this point. This calendar system allows day numbers of -4,000,000,000 through 4,000,000,000.
The Feasts where holidays prescribed in the Law of Moses.
The definition of the calendar given above suggests when those feasts should happen in the calendar year. One set, covered below, happened based on the harvest, something asynchronous to the calendar.
The following lists each calendar based event and the date on which it falls.
On X/1/10, the 10th day of the 1st month, the Israelites were to select the lamb tht they intended to slaughter at Passover.
At the end of the X/1/14, at twilight, when the sun was setting and the day was essentially over, the Israelites were to sacrifice the Passover Lamb. (Exodus 12:6)
This event is given more precisely than simply to the day, The time of day within the day was also given.
When marked on the calendar, the 14th is the day so marked, but the time of day that was important to this holiday was the time when the sun was setting, in the late afternoon.
Generally speaking, this 14th day of the first month was a regular day so that preparations for the Passover Meal could be made during the day.
On X/1/15 starting after sunset on Passover, the day before, the Israelites where to have a special holiday. This first night was a night for an all night vigil.
This vigil commemorates the night when the first born of Egypt were slain at midnight.(Exodus 12:29)
A sacred assembly was to be held on this day.
This day, the 15th, and all days through and including X/1/21 were to be the feast of unleavened bread. No food made with yeast was to be eaten during this time.
On X/1/21 there was to be a closing sacred assembly.
On X/7/1 was Trumpets. (Lev. 23:23)
Day Of Atonement
On X/7/10 was the Day of Atonement. (Lev. 16:29, Lev. 23:26)
Beginning on X/7/15 and running through and including X/7/22 was the time of Booths.
There was a Sacred Assembly on X/7/22.
First fruits is not based on the calendar, but on the harvest of new grain. As such, this holiday is based on the solar year and the location on the planet were the observer lives. This means that generally, people living in the southern hemisphere observe this holiday about 1/2 of a solar year away from those in the northern hemisphere.
This holiday also has annual variability based on each year’s particular climate conditions, and generally falls later in the solar year for locations located in climates further away from the equator.
This variability in the time of the associated holiday reflected the way the Israelites followed the pillar of fire in the wilderness. They ever knew until the last minute if they would head out, or say, as they were led through the wilderness.
So, too, this holiday was unpredictable. It wasn’t possible to accurately predict when it would fall on a year to year basis.
First Fruits is based on the wheat harvest, (though some say Barley).
The purpose of this holiday was to give thanks for the wheat, the bread, and to provide a special day of rest before commencing the grueling task of harvesting the wheat.
Since in history this would have been for the wheat you eat the modern mechanized farming operations, as well as world wide distribution of wheat, poses problems for modern observation. The United States is a supplier of wheat to over 100 other nations, so it is possible for the United States wheat operations to be a possible trigger for this holiday.
For information on the exact timing of wheat harvest in the United States see the Wheat Harvest reports generated weekly at www.uswheat.org. These reports generally start in June and run into September, the time of the wheat harvest in the United States.
Wheat is said to "turn" a few days before harvest begins which is probably a modern reflection of the Biblical mandate to harvest an initial sheaf and then wait until the day after the next Sabbath. On that day the sheaf is to be waved and then the harvest commences on the next day. This inserts about a 4 day delay from the turning of the wheat until the full harvest commences.
The first day of the week after the 7th full week from that first day is the end of first fruits and is the date for New Grain.