Bible prophecy involves times, dates, days, weeks, months and years. This necessarily means the use of calendars. There are a vast number of different calendars used by different peoples in different times. The Gregorian calendar is the calendar used in most parts of the world. It was introduced by Pope Gregory XIII in 1582. To understand calendars we have to understand about the Earth's rotation around the sun.
The Solar Year
A solar year is, on average, approximately 365 days, 5 hours, 48 minutes and 45 seconds long (365.24219 days). This creates problems for calendar makers because it is not a round number of days. Over the centuries people have got round this problem in various ways in order to keep their calendars in line with the seasons.
The Gregorian Calendar
Our current calendar is the Gregorian Calendar. This has a 365 day year but, as a year is slightly more than this, an extra day is added every fourth year which is called a leap year. If a year was exactly 365.25 days then this would work. However as it is 365.24219 days it does not quite work so a further adjustment has to be made. Years that are divisible by a hundred are not leap years unless they are divisible by four hundred in which case they are. Consequently 1900 for example was not a leap year but the year 2000 was a leap year. This system keeps the calendar nearly enough in line with the seasons.
The Julian Calendar
The Julian Calendar was originally proposed by Julius Caesar and was the calendar we had before the Gregorian Calendar. The Calendar simply took a year as 365.25 days and allowed for one leap year every four years. Over the centuries the slight discrepancy from the actual year length led to an increasing misalignment of the calendar with the seasons. For this reason Pope Gregory introduced his reform and most Catholic countries soon followed. Britain and its colonies did not change until much later.
The Calendar (New Style) Act 1750 introduced the Gregorian calendar to the British Empire, bringing Britain into line with most of Western Europe. The introduction of the new calendar was not straightforward. It meant that the year 1751 was a short year, lasting just 282 days from 25th March (New Year in the Julian calendar) to 31st December. The year 1752 then began on 1 January. There remained the problem of aligning the calendar in use in England with that in use in Europe. It was necessary to correct it by 11 days: the ‘lost days’. It was decided that Wednesday 2nd September 1752 would be followed by Thursday 14th September 1752.
The Jewish Calendar
The Jewish calendar is based on the lunar year of 354 days. The 12 months of the calendar are alternatively 30 days and 29 days making 354 days altogether. Obviously this is way off the actual length of a solar year so intercalary months are inserted in the calendar to harmonise with the solar year. Modern Jewish calendars generally show that in the third, sixth and eighth years an intercalary month is inserted in order to keep the first month, Nisan, in the Spring season. However there is no extra month mentioned in the bible. The 13th month was simply a gap of 30 or 31 days that was left while Israel waited to see the New Moon which signaled their New Year. This had to be when the barley was ready. The first month was originally called Abib (green ears) and only later called Nisan. Since bible times the Jews have brought in their system of intercalary months but this often positions the first month of Nisan too early in the Spring. Frank Paine, in his book "The Miracle of Time" believed the 354 day lunar calendar to be one of the keys to understanding God's plan of time. See the section on "The Jubilee Cycle" for more details.
The 360 Day Calendar
There is evidence of a 360 day year in the bible. See section on "The 360 Day Prophetic Year" for more details. This measure of the year seems to be of use in prophecy and determining God's prophetic plan rather than being of use as a calendar for practical purposes. Sir Robert Anderson believed the 360 day year to be a key to understanding bible chronology. See the section on "Sir Robert Anderson" for more details. A 360 day year is made up of 12 months of 30 days each and so has a neatness and perfection to it that is more heavenly than earthly.
The 364 Day Calendar
There are many other calendars but one that is worth a mention is the 364 day calendar. This calendar was found during the discovery of the Dead Sea Scrolls. Tens of thousands of Dead Sea Scroll fragments were discovered in the Qumran caves near the Dead Sea. These scrolls were first discovered in 1946 but it has taken a long time for them to be studied and deciphered. Scroll 4Q324d describes a 364 day calendar used by the Qumran community that lived in the Judean Desert. This Qumran calendar gives us insight into how the community organized the seasons and religious festivals. Whereas the lunar calendar required astronomical observations and human decisions regarding the timing of months and festivals the 364 day calendar was regular. It could be divided into exactly 52 weeks and was also divisible by 4. This meant that special occasions could always fall on the same day. So for example Passover was always on Tuesday the 14th of Nisan. The calendar was kept in alignment by having a leap week every five to six years. This calendar was the one used by the Essenes, the Jewish sect that were around at the time of Jesus. They believed that the 354 day lunar calendar used by the pharisees was sacrilegious and meant that the religious festivals were being celebrated on the wrong days.