What we take for granted in our daily lives is the structure of calendars and the system of time that people in one particular place or another follow.

For Europe and the US, it is the Gregorian solar calendar of 365 days (366 days in a leap year) with January 1st being the first day of the year. For the Middle East there is the Hijri lunar calendar made of 354 days where Muhram 1st is the first day of the year. For the Jewish faith, the calendar is a luni-solar one where the beginning of the calendar is around October. For the Chinese it is luni-solar, for the Indians it is solar, etc.

As you can see above, there is a mish-mash of systems being followed, each having its own criteria and each having its own start date and details...

Is the year solar or lunar?

The first question that needs to be asked when constructing any calendar is whether to use a solar year of 365 days (representing the length of time it takes the Earth to rotate completely around the sun), or, whether to use a lunar year by simply counting 12 months (354 days), or, whether to use a mixture of both solar and lunar at the same time.

"And We made the night and the day as two signs, so We erased the sign of night and We made the sign of day to see-in, that you may seek bounty from your Lord, and that you may know the number of the years and the count. And everything We have detailed completely." (17:12)

"He is the One who made the sun to emit light, and the moon to reflect it, and He measured its phases; that you may know the number of the years and the count. God has not created this except for truth. He details the revelations for a people who know. (10:5)

According to the above verses, the main theme that is used to measure the year is the alternation of the day and night (which is caused by the movement of the sun), while the secondary theme are the phases of the moon, thus making the calendar that is described in God's Scripture as being 'luni-solar' in nature.

Year = 365 days rounded (365.24 days - one full rotation of the earth around the sun)

Month

With regards to the definition of a 'month', it is a unit of measure that is contained within a year (already established as 365 days). According to the Scripture, there are 12 of these units 'months' that are counted within the system of the year:

"The count of the months with God is twelve months in God's record the day He created the heavens and the Earth; four of them are restricted. This is the correct system; so do not wrong yourselves in them; and fight those who set up partners collectively as they fight you collectively. And know that God is with the righteous." (9:36)

Therefore, and in the simplest terms, the average length of each month is 365 ÷ 12 = 30.4 days (30 days rounded).

This figure of 30 days brings us back to the moon whereby we are told that, in addition to the sun, the moon is also related to the count of the years.

"He is the One who made the sun to emit light, and the moon to reflect it, and He measured its phases; that you may know the number of the years and the count. God has not created this except for truth. He details the revelations for a people who know. (10:5)

The average length of the cycle of the moon is 29.53 days (30 days rounded), which is in-line with the measure of the 'month' as given in the Scripture.

Month = 30 days rounded (29.53 days - one full lunar cycle)

When does the month begin?

Due to the unique phases that accompany the cycle of the moon (dark moon, to crescent, to full moon, to crescent, to dark moon) we need to examine the Scripture to determine which of these phases is used to mark the beginning of any given month.

"They ask you regarding the crescent moons, Say: "They are a timing mechanism for the people as well as for the Pilgrimage." And piety is not that you would enter a home from its back, but piety is whomever is righteous and come to the homes from their main doors. And be aware of God that you may succeed." (2:189)

We know that the subject of the months was already known by the people (as evidenced by the Scripture speaking of it being revealed in the `month of Ramadhan`), therefore, the above verse discounts the crescent moons as being the beginning of the month due to the fact that the prophet was being `asked` about  what the crescent moons signify (the questions people asked and were recorded in the Scripture were all ones where the people were unsure/unclear: what about the soul? What about sex during menstruation? What about the hour? etc.).

We are now left with two options for the beginning of the month: dark moon or full moon.

"And the moon We have measured it to appear in stages, until it returns to being like an old curved sheath." (36:39)

The `old curved sheath` of a palm tree is the section of branch that dies and then hangs down in a curved shape on either side of the tree.

The only time we can see this phenomena with the moon is during a `full moon` when the darkness of the night sky makes a curved shape on the left and on the right (like an old curved sheath which has died and changed its color from the original tree).

Therefore, the months begins with the full moon.

When does the year begin?

The current dates used in modern calendars to mark the first day of the year (such as January 1 in the Gregorian calendar or Muharam 1 in the Arabic calendar) are arbitrary dates that have been selected based on specific historic events (the death of Jesus in the Gregorian, and the Hijra of the Prophet in the Arabic).

Scientifically speaking, the solar year has 4 clear markers that are caused by the tilt of the Earth and its position vis-à-vis the sun which recur each year around the same time:

Spring Equinox (when the length of the day and night are exactly equal)

Fall Equinox (when the length of the day and night are exactly equal)

Summer Solstice (when the length of the day is greatest).

Winter Solstice (when the length of the night is greatest).

These markers are quite reliable in the sense that both animal and plant species react to these changes by adapting their behavior accordingly. Please note that we are looking to the seasons in relation to the northern hemisphere since the Scripture was revealed in the area of the Middle East, which lies in the northern hemisphere.

The most logical application of a luni-solar calendar is to decide which of these 'fixed' dates is to be the marker and then begin counting the first full moon that occurs on or after this date as day-1, month-1.

Looking to the Scripture for guidance, we find that there is a clear evidence given to the "Winter Solstice" as being the blessed "Night of Decree" (see article). Thus, the month in which this blessed night falls would be the 1st month of the Islamic Calendar.

"The month of Ramadhan, in which the Qur’an was revealed; as a guide to the people and a clarification of the guidance and the criterion. Therefore, whoever of you witnesses the month, then let him fast therein. And whoever is ill or traveling, then the same number from different days. God wants to bring you ease and not to bring you hardship; and so that you may complete the count, and glorify God for what He has guided you, that you may be thankful." (2:185)

This day would also mark the month of Ramadhan which heralds a period of limited fasting.

The 13th month?

Having a luni-solar year is extremely accurate when it comes to aligning the year with the seasons, planning for crops and harvests, and in knowing the natural cycle of wildlife. However, one issue that needs to be addressed is the '13th month' which occurs in some solar years.

Due to the length of a lunar month being on average 29.54 days, this means that it is possible to have a 13th full moon appear within the 365 day year.

The problem is not in having this extra month appear (which is a natural phenomena of the length of the moon's cycle within the length of the solar year), but in how the extra month is dealt with.

"Know that the use of the additional month causes an increase in rejection, for it is used by those who have rejected that they may misguide with it by making it lawful one year and forbidding it one year, so as to circumvent the count that God has made restricted; thus they make lawful what God made forbidden! Their evil works have been adorned for them, and God does not guide the rejecting people." (9:37)

The Scripture tells us that previous generations had manipulated the 13th month by counting it in some years and completely ignoring it in others, thereby circumventing the correct start of the restricted months in-order to make the hunting of wildlife lawful in the period that God has decreed was 'forbidden'.

To address this issue, the Scripture decreed the simplest answer, which is to simply ignore the 13th month that occurs within a solar year and thus keeping with the instruction that only 12 months are to be maintained within the solar year:

"The count of the months with God is twelve months in God's record the day He created the heavens and the Earth; four of them are restricted. This is the correct system; so do not wrong yourselves in them; and fight those who set up partners collectively as they fight you collectively. And know that God is with the righteous." (9:36)

Therefore, any new full moon beyond 12 new full moons that appears within a solar year (i.e. from winter solstice to winter solstice) will be ignored in the year count and treated as 'month zero'.

Conclusion

God's calendar is one that exists in nature and which utilizes the entities of sun, moon and star to make a perfect timing mechanism that is in-line with nature and in-harmony with our movements through the universe.

Below is a summary of what has been explained above:

The year according to the Scripture is Luni-Solar (17:12, 10:5).

The months begin with the full moon (2:189, 36:39).

The solar year has 4 markers that can be measured (2 solstices, 2 equinoxes).

The most special event that occurs during the year is the Night of Decree (97:4) which occurs on the winter solstice.

The month of the winter solstice is known as the month of "Ramadhan" (2:185).

After a count of 12 lunar months, if a 13th full moon appears before the winter solstice, then such a month is to be ignored and counted as 'month zero'. This average occurrence of this extra month is 7 times every 19 solar years.