A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates. The traditional concept arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today, and are used to divide the year.
The following types of months are mainly of significance in astronomy, most of them (but not the distinction between sidereal and tropical months) first recognized in Babylonian lunar astronomy.
A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. The Sun moves eastward with respect to the stars (as does the Moon) and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun.
An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years. Therefore, the Moon takes a little longer to return to perigee than to return to the same star.
A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18.6 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star.
At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 solar days: a 30-day full month followed by a 29-day hollow month — but this is only roughly accurate and regularly needs intercalation (correction) by a leap day.
Additionally, the synodic month does not fit easily into the solar (or 'tropical') year, which makes accurate, rule-based lunisolar calendars that combine the two cycles complicated. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years (which add up to not quite 6,940 days): 12 years have 12 lunar months, and 7 years are 13 lunar months long. However, a Metonic calendar based year will drift against the seasons by about one day every 2 centuries. Metonic calendars include the calendar used in the Antikythera Mechanism about 21 centuries ago, and the Hebrew calendar.
Alternatively in a pure lunar calendar, years are defined as having always 12 lunations, so a year is 354 or 355 days long: the Islamic calendar is the prime example. Consequently, an Islamic year is about 11 days shorter than a solar year and cycles through the seasons in about 33 solar = 34 lunar years: the Islamic New Year has a different Gregorian calendar date in each (solar) year.
Purely solar calendars often have months which no longer relate to the phase of the Moon, but are based only on the motion of the Sun relative to the equinoxes and solstices, or are purely conventional like in the widely used Gregorian calendar.
The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunisolar and lunar calendars for civil use in most societies.
The Hellenic calendars, the Hebrew Lunisolar calendar and the Islamic Lunar calendar started the month with the first appearance of the thin crescent of the new moon.
However, the motion of the Moon in its orbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore, the beginning and lengths of months defined by observation cannot be accurately predicted.
While some like orthodox Islam and the Jewish Karaites still rely on actual moon observations, reliance on astronomical calculations and tabular methods is increasingly common in practice.
Roman calendar was reformed several times, the last three enduring reforms during historical times. The last three reformed Roman calendars are called the Julian, Augustan, and Gregorian; all had the same number of days in their months. Despite other attempts, the names of the months after the Augustan calendar reform have persisted, and the number of days in each month (except February) have remained constant since before the Julian reform. The Gregorian calendar, like the Roman calendars before it, has twelve months, whose Anglicized names are:
29 in leap years
|7|| July |
|8|| August |
The famous mnemonic Thirty days hath September is a common way of teaching the lengths of the months in the English-speaking world. The knuckles of the four fingers of one's hand and the spaces between them can be used to remember the lengths of the months. By making a fist, each month will be listed as one proceeds across the hand. All months landing on a knuckle are 31 days long and those landing between them are 30 days long, with variable February being the remembered exception. When the knuckle of the index finger is reached (July), go over to the first knuckle on the other fist, held next to the first (or go back to the first knuckle) and continue with August. This physical mnemonic has been taught to primary school students for many decades, if not centuries.
This cyclical pattern of month lengths matches the musical keyboard alternation of wide white keys (31 days) and narrow black keys (30 days). The note F corresponds to January, the note F♯ corresponds to February, the exceptional 28–29 day month, and so on.
The mean month-length in the Gregorian calendar is 30.436875 days.
Any five consecutive months, that do not include February, contain 153 days.
Months in the pre-Julian Roman calendar included:
The Romans divided their months into three parts, which they called the calends, the nones, and the ides. Their system is somewhat intricate. The ides occur on the thirteenth day in eight of the months, but in March, May, July, and October, they occur on the fifteenth. The nones always occur 8 days (one Roman ‘week’) before the ides, i.e., on the fifth or the seventh. The calends are always the first day of the month, and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).
Within a month, the following dates fall on the same day of the week:
Some months have the same date/weekday structure.
In a non-leap year:
In a leap year:
The Hebrew calendar has 12 or 13 months.
Adar 1 is only added 7 times in 19 years. In ordinary years, Adar 2 is simply called Adar.
There are also twelve months in the Islamic calendar. They are named as follows:
See Islamic calendar for more information on the Islamic calendar.
|Gregorian month||Arabic month|
|January||يناير||كانون الثاني||Kanun Al-Thani|
|October||أكتوبر||تشرين الأول||Tishrin Al-Awwal|
|November||نوفمبر||تشرين الثاني||Tishrin Al-Thani|
|December||ديسمبر||كانون الأول||Kanun Al-Awwal|
The Hindu calendar has various systems of naming the months. The months in the lunar calendar are:
|Sanskrit name||Tamil name||Telugu name||Nepali name|
|1||Chaitra (चैत्र)||Chitirai (சித்திரை)||Chaithramu (చైత్రము)||Chaitra (चैत्र/चैत)|
|2||Vaiśākha (वैशाख)||Vaikasi (வைகாசி)||Vaisaakhamu (వైశాఖము)||Baisakh (बैशाख)|
|3||Jyeṣṭha (ज्येष्ठ)||Aani (ஆனி)||Jyeshttamu (జ్యేష్ఠము)||Jesth (जेष्ठ/जेठ)|
|4||Ashadha (आषाढ)||Aadi (ஆடி)||Aashaadhamu (ఆషాఢము)||Aasad (आषाढ/असार)|
|5||Śrāvaṇa (श्रावण)||Aavani (ஆவணி)||Sraavanamu (శ్రావణము)||Srawan (श्रावण/साउन)|
|6||Bhadrapada (भाद्रपद)||Purratasi (புரட்டாசி)||Bhaadhrapadamu (భాద్రపదము)||Bhadau (भाद्र|भदौ)|
|7||Āśvina (अश्विन)||Aiypasi (ஐப்பசி)||Aasveeyujamu (ఆశ్వయుజము)||Asoj (आश्विन/असोज)|
|8||Kārtika (कार्तिक)||Kaarthigai (கார்த்திகை)||Kaarthikamu (కార్తీకము)||Kartik (कार्तिक)|
|9||Mārgaśīrṣa (मार्गशीर्ष)||Maargazhi (மார்கழி)||Maargaseershamu (మార్గశిరము)||Mangsir (मार्ग/मंसिर)|
|10||Pauṣa (पौष)||Thai (தை)||Pushyamu (పుష్యము)||Push (पौष/पुष/पूस)|
|11||Māgha (माघ)||Maasi (மாசி)||Maaghamu (మాఘము)||Magh (माघ)|
|12||Phālguna (फाल्गुन)||Panguni (பங்குனி)||Phaalgunamu (ఫాల్గుణము)||Falgun (फाल्गुन/फागुन)|
These are also the names used in the Indian national calendar for the newly redefined months. Purushottam Maas or Adhik Maas (translit. adhika = 'extra', māsa = 'month') is an extra month in the Hindu calendar that is inserted to keep the lunar and solar calendars aligned. "Purushottam" is an epithet of Vishnu, to whom the month is dedicated.
The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are
The Baháʼí calendar is the calendar used by the Baháʼí Faith. It is a solar calendar with regular years of 365 days, and leap years of 366 days. Years are composed of 19 months of 19 days each (361 days), plus an extra period of "Intercalary Days" (4 in regular and 5 in leap years).The months are named after the attributes of God. Days of the year begin and end at sundown.
The Iranian / Persian calendar, currently used in Iran and Afghanistan, also has 12 months. The Persian names are included in the parentheses. It begins on the northern Spring equinox.
The Bangla calendar, used in Bangladesh, follows solar months and it has six seasons. The months and seasons in the calendar are:
|No.||Name (Bengali)||Name (Sylheti)||Name (Rohingya)||Season||Days||Roman months|
|1||Boishakh (বৈশাখ)||Boishakh||Boicák||Grishmo (গ্রীষ্ম)||31||14 April – May|
|2||Joishtho (জ্যৈষ্ঠ)||Zoit||Zeth||Grishmo (গ্রীষ্ম)||31||May – June|
|3||Asharh (আষাঢ়)||Aaŗ||Acár||Borsha (বর্ষা)||31||June – July|
|4||Shrabon (শ্রাবণ)||Haon||Cón||Borsha (বর্ষা)||31||July – August|
|5||Bhadro (ভাদ্র)||Bhado||Bádo||Shorot (শরৎ)||31||August – September|
|6||Aashin (আশ্বিন)||Ashin||Acín||Shorot (শরৎ)||30||September – October|
|7||Kartik (কার্তিক)||Khati||Hati||Hemonto(হেমন্ত)||30||October – November|
|8||Ogrohayon (অগ্রহায়ণ)||Aghon||Óon||Hemonto(হেমন্ত)||30||November – December|
|9||Poush (পৌষ)||Phush||Fuc||Sheet (শীত)||30||December – January|
|10||Magh (মাঘ)||Magh (মাঘ)||Mak||Sheet (শীত)||30||January – February|
|11||Falgun (ফাল্গুন)||Fagun||Fóon||Boshonto (বসন্ত)||30 (31 in leap years)||February – March|
|12||Choitro (চৈত্র)||Soit||Soit||Boshonto (বসন্ত)||30||March – April|
The months in the Nanakshahi calendar are:
|1||Chet||ਚੇਤ||31||14 March – 13 April|
|2||Vaisakh||ਵੈਸਾਖ||31||14 April – 14 May|
|3||Jeth||ਜੇਠ||31||15 May – 14 June|
|4||Harh||ਹਾੜ||31||15 June – 15 July|
|5||Sawan||ਸਾਵਣ||31||16 July – 15 August|
|6||Bhadon||ਭਾਦੋਂ||30||16 August – 14 September|
|7||Assu||ਅੱਸੂ||30||15 September – 14 October|
|8||Katak||ਕੱਤਕ||30||15 October – 13 November|
|9||Maghar||ਮੱਘਰ||30||14 November – 13 December|
|10||Poh||ਪੋਹ||30||14 December – 12 January|
|11||Magh||ਮਾਘ||30||13 January – 11 February|
|12||Phagun||ਫੱਗਣ||30/31||12 February – 13 March|
Different from the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar. The solar is used more commonly than the lunar calendar. There are 12 months and the numbers of days follow the Julian and Gregorian calendar.
|Gregorian month||Meaning||Zodiac sign|
|January||មករា||Môkâréa||មករ môkâr; "naga"||Capricorn|
|February||កុម្ភៈ||Kŏmpheă||ក្អម k’âm; "clay pitcher"||Aquarius|
|March||មិនា/មីនា||Mĭnéa/Minéa||ត្រី trei; "fish"||Pisces|
|April||មេសា||Mésa||ចៀម chiĕm; "sheep"||Aries|
|May||ឧសភា||Ŭsâphéa||គោឈ្មោល koŭ chhmoŭl; "bull"||Taurus|
|June||មិថុនា||Mĭthŏnéa||គូ ku; "couple"||Gemini|
|July||កក្កដា||Kâkkâda||ក្ដាម kdam; "crab"||Cancer|
|August||សីហា||Seiha||សីហៈ seihă; "lion"||Leo|
|September||កញ្ញា||Kănhnha||ក្រមុំ krâmŭm; "maiden"||Virgo|
|October||តុលា||Tŏléa||ជញ្ជីង chônhching; "scales"||Libra|
|November||វិច្ឆិកា||Vĭchchhĕka||ខ្ទួយ khtuŏy; "scorpion"||Scorpio|
|December||ធ្នូ||Thnu||ធ្នូ thnu; "bow, arc"||Sagittarius|
The Khmer lunar calendar contains 12 months; however, the eighth month is repeated (as a "leap month") every two or three years, making 13 months instead of 12.
|English name||Thai name||Abbr.||Transcription||Sanskrit word||Zodiac sign|
|February||กุมภาพันธ์||ก.พ.||kumphaphan||kumbha "pitcher, water-pot"||Aquarius|
|March||มีนาคม||มี.ค.||minakhom||mīna "(a specific kind of) fish"||Pisces|
|June||มิถุนายน||มิ.ย.||mithunayon||mithuna "a pair"||Gemini|
|December||ธันวาคม||ธ.ค.||thanwakhom||dhanu "bow, arc"||Sagittarius|
The Tongan calendar is based on the cycles of the moon around the earth in one year. The months are:
Pingelapese, a language from Micronesia, also uses a lunar calendar. There are 12 months associated with their calendar. The moon first appears in March,[ clarification needed ] they name this month Kahlek. This system has been used for hundreds of years and throughout many generations. This calendar is cyclical and relies on the position and shape of the moon.
|Malayalam name||Transliteration||Concurrent Gregorian months||Sanskrit word and meaning||Zodiac sign|
|ധനു||dha-nu||December–January||dhanu "bow, arc"||Sagittarius|
|കുംഭം||kum-bha-m||February–March||kumbha "pitcher, water-pot"||Aquarius|
|മീനം||mee-na-m||March–April||mīna "(a specific kind of) fish"||Pisces|
|ഇടവം||i-Ta-va-m||May – June||vṛṣabha "bull"||Taurus|
|മിഥുനം||mi-thu-na-m||June–July||mithuna "a pair"||Gemini|
The Sinhalese calendar is the Buddhist calendar in Sri Lanka with Sinhala names. Each full moon Poya day marks the start of a Buddhist lunar month.The first month is Vesak.
The old Icelandic calendar is not in official use anymore, but some Icelandic holidays and annual feasts are still calculated from it. It has 12 months, broken down into two groups of six often termed "winter months" and "summer months". The calendar is peculiar in that the months always start on the same weekday rather than on the same date. Hence Þorri always starts on a Friday sometime between January 22 and January 28 (Old style: January 9 to January 15), Góa always starts on a Sunday between February 21 and February 27 (Old style: February 8 to February 14).
|Month||Georgian month name||Transliteration||Georgian other names||Transliteration|
|January||აპნისი, აპანი||Apnisi, Apani|
|June||მარიალისა||Marialisa||თიბათვე, ივანობისთვე||Tibatve, Ivanobistve|
|July||თიბისა||Tibisa||მკათათვე, კვირიკობისთვე||Mkatatve, Kvirikobistve|
|November||ტირისკონი||Tiriskoni||გიორგობისთვე, ჭინკობისთვე||Giorgobistve, Chinkobistve|
*NOTE:New Year in ancient Georgia started from September.
Like the Old Norse calendar, the Anglo-Saxons had their own calendar before they were Christianized which reflected native traditions and deities. These months were attested by Bede in his works On Chronology and The Reckoning of Time written in the 8th century. His Old English month names are probably written as pronounced in Bede’s native Northumbrian dialect. The months were named after the moon; the new moon marking the end of an old month and start of a new month; the full moon occurring in the middle of the month, after which the whole month took its name.
|1||Æfterra-ġēola mōnaþ||“After-Yule month”||January|
|5||Ðrimilce-mōnaþ||“Three-milkings month” ||May|
|9|| Hāliġ-mōnaþ or|
|“Holy month” or|
When an intercalary month was needed, a third Litha month was inserted in mid-summer.
Nagyszombati kalendárium (in Latin: Calendarium Tyrnaviense) from 1579. Historically Hungary used a 12-month calendar that appears to have been zodiacal in naturebut eventually came to correspond to the Gregorian months as shown below:
The ancient civil Egyptian calendar had a year that was 365 days long and was divided into 12 months of 30 days each, plus 5 extra days (epagomenes) at the end of the year.The months were divided into 3 "weeks" of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events "wandered" through the calendar, it is referred to as Annus Vagus or "Wandering Year".
The Nisga'a calendar coincides with the Gregorian calendar with each month referring to the type of harvesting that is done during the month.[ citation needed ]
This calendar was proposed during the French Revolution, and used by the French government for about twelve years from late 1793. There were twelve months of 30 days each, grouped into three ten-day weeks called décades. The five or six extra days needed to approximate the tropical year were placed after the months at the end of each year. A period of four years ending on a leap day was to be called a Franciade. It began at the autumn equinox:
Ojibwe month names 27) that marks when sunrise is the latest in the Northern Hemisphere.are based on the key feature of the month. Consequently, months between various regions have different names based on the key feature of each month in their particular region. In the Eastern Ojibwe, this can be seen in when the sucker makes its run, which allows the Ojibwe to fish for them. Additionally, Rhodes also informs of not only the variability in the month names, but how in Eastern Ojibwe these names were originally applied to the lunar months the Ojibwe originally used, which was a lunisolar calendar, fixed by the date of Akiinaaniwan (typically December
|Original order in the Ojibwa year||Starting at the first full moon after:|
in those places that have a sucker run during that time
|Akiinaaniwan on December 27|
in those places that have a sucker run during that time
in those places that do not have a sucker run during that time
in those places that have an April sucker run
in those places that have a January sucker run
in those places that have an April sucker run
in those places that have a January sucker run
in those places that do not have a sucker run during that time
|[o]shki-b[i]boon-gii[zi]soons||Little new-winter moon|
|only used if the new moon after g[i]chi-b[i]boon-giizis occurs before Akiinaaniwan on December 27.|
A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single, specific day within such a system. A calendar is also a physical record of such a system. A calendar can also mean a list of planned events, such as a court calendar or a partly or fully chronological list of documents, such as a calendar of wills.
The traditional Chinese calendar, is a lunisolar calendar which identifies years, months, and days according to astronomical phenomena. In China, it is defined by the Chinese national standard GB/T 33661–2017, "Calculation and Promulgation of the Chinese Calendar", issued by the Standardization Administration of China on May 12, 2017.
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon. This means that the lunar hemisphere facing Earth—the near side—is completely sunlit and appears as an approximately circular disk. The full moon occurs roughly once a month.
The Hebrew calendar, also called the Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits, and daily Psalm readings, among many ceremonial uses. In Israel, it is used for religious purposes, provides a time frame for agriculture, and is an official calendar for civil holidays, alongside the Gregorian calendar.
Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.
A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting an additional day or month into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected. A year that is not a leap year is a common year.
A lunar calendar is a calendar based on the monthly cycles of the Moon's phases, in contrast to solar calendars, whose annual cycles are based only directly on the solar year. The most commonly used calendar, the Gregorian calendar, is a solar calendar system that originally evolved out of a lunar calendar system. A purely lunar calendar is also distinguished from a lunisolar calendar, whose lunar months are brought into alignment with the solar year through some process of intercalation. The details of when months begin vary from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.
A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In this case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.
The Metonic cycle or enneadecaeteris is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates the construction of a lunisolar calendar.
In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse.
A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.
A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other type of calendar is a lunar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.
The Hindu calendar, Panchanga or Panjika is one of various lunisolar calendars that are traditionally used in the Indian subcontinent and Southeast Asia, with further regional variations for social and Hindu religious purposes. They adopt a similar underlying concept for timekeeping based on sidereal year for solar cycle and adjustment of lunar cycles in every three years, but differ in their relative emphasis to moon cycle or the sun cycle and the names of months and when they consider the New Year to start. Of the various regional calendars, the most studied and known Hindu calendars are the Shalivahana Shaka(Based on the King Shalivahana, also the Indian national calendar) found in the Deccan region of Southern India and the Vikram Samvat (Bikrami) found in Nepal and the North and Central regions of India – both of which emphasize the lunar cycle. Their new year starts in spring. In regions such as Tamil Nadu and Kerala, the solar cycle is emphasized and this is called the Tamil calendar and Malayalam calendar and these have origins in the second half of the 1st millennium CE. A Hindu calendar is sometimes referred to as Panchangam (पञ्चाङ्ग), which is known also known as Panjika in Eastern India.
The epact, used to be described by medieval computists as the age of a phase of the Moon in days on 22 March; in the newer Gregorian calendar, however, the epact is reckoned as the age of the ecclesiastical moon on 1 January. Its principal use is in determining the date of Easter by computistical methods. It varies from year to year, because of the difference between the solar year of 365–366 days and the lunar year of 354–355 days.
Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.
The Buddhist calendar is a set of lunisolar calendars primarily used in Cambodia, Laos, Myanmar, India, Sri Lanka, and Thailand as well as in Malaysia, Singapore and Vietnam by Chinese populations for religious or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunar calendar and by the Thai solar calendar.
The Burmese calendar is a lunisolar calendar in which the months are based on lunar months and years are based on sidereal years. The calendar is largely based on an older version of the Hindu calendar, though unlike the Indian systems, it employs a version of the Metonic cycle. The calendar therefore has to reconcile the sidereal years of the Hindu calendar with the Metonic cycle's near tropical years by adding intercalary months and days at irregular intervals.
Adhik Maas is an extra month in the Hindu calendar that is inserted to keep the lunar and solar calendars aligned.
Nisan-years is an ancient calendar system used around Mesopotamia. Its beginning was from the prehistorical era. Ever since Mesopotamia had historical writings, even before the First Babylonian dynasty of Hammurabi, its calendar used the Nisan-years.
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.