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The International Lunar Calendar or International Lunation Calendar (ILC) is an optional subset of the International Calendar (IC), hence a standardized variant of the proleptic Gregorian calendar (PGC). It runs in parallel with the International Month Calendar (IMC), the International Week Calendar (IWC), the International Day Calendar (IDC) and the International Season Calendar (IAC/IZC).


The lunation year has 354 days in a common year and either 383 in a short leap year or 384 days in a long leap year. It’s divided into 12 months of systematic, alternating length: 30 and 29 days with certain exceptions. Weeks are not used with the IMC. The date notation specified in IC is commonly used for the ILC, but is not a mandatory part of it.


The lunar year contains either 12 or 13 complete lunar months. A lunar month or lunation has 29 or 30 days. Although it can be astronomically defined exactly from new moon to new moon, e.g. at a reference point like 0° 0°, with a day belonging to the lunation the majority of its hours belong to, the 12 common months of the International Lunar Calendar alternate between 30 and 29 days. Only the 13th, leap month, conveniently positioned at the end of the year, can differ in length: it has either 29 or 30 days in order to synchronize with astronomic reality. A lunar month belongs to the month year the majority of its days (15 to 30) fall into; in the unlikely case of a 30-day leap month with 15 days in both years it belongs to the year most of its weeks belong to.

Equivalencies in traditional lunisolar calendars[]

International lunar month lengths compared to cognates
ILC Hebrew Buddhist
Label Days Short Long Early
short
Early
long
Common Leap
L01 30 29 29 29 29 30 30
L02 29 30 30 29 29 29 29
L03 30 30 29 30 30 30 30
L04 29 29 30 30 29 29 29
L05 30 30 29 29 30 30 30
L06 29 29 30 30 29 29/30 29/30
L07 30 30 29 29 30 30 30
L08 29 29 30 30 29 29 30
L09 30 30 29 29 30 30 29
L10 29 29/30 30 30 29 29 30
L11 30 30/29 29/30 29/30 30 30 29
L12 29 29 30/29 30/29 29/30 29 30
L13 30/29 29 30/29 29
Σ 354;
383–384
353–355 382–384 353–355 382–384 354–355 384–385


International lunar months ordinal numbering equivalents
in common 12-month years
ILC Hebrew Chinese Buddhist
Biblical Civil Modern Earthly Civil Traditional
L01 11 05 12 02 10 02 03 04
L02 12 06 01 03 11 03 04 05
L03 01 07 02 04 12 04 05 06
L04 02 08 03 05 01 05 06 07
L05 03 09 04 06 02 06 07 08
L06 04 10 05 07 03 07 08 09
L07 05 11 06 08 04 08 09 10
L08 06 12 07 09 05 09 10 11
L09 07 01 08 10 06 10 11 12
L10 08 02 09 11 07 11 12 01
L11 09 03 10 12 08 12 01 02
L12 10 04 11 01 09 01 02 03
Offset −2 +4 −1 +1 −3 +1 +2 +3
International lunar months ordinal numbering equivalents
in leap 13-month years
ILC Hebrew Chinese Buddhist
Biblical Civil Modern Earthly Civil Traditional
L01 11 05 12 02 10 02 03 04
L02 12 06 01 03 11 03 04 05
L03 01 07 02 04 12 04 05 06
L04 02 08 03 05 01 05 06 07
L05 03 09 04 06 02 06 07 08
L06 04 10 05 07 03 07 08 09
L07 05 11 06 08 04 08 09 10
L08 06 12 07 09 05 09 10 11
L09 07 01 08 10 06 10 11 12
L10 08 02 09 11 07 11 12 13
L11 09 03 10 12 08 12 13 01
L12 10 04 11 13 09 13 01 02
L13 11 05 12 01 10 01 02 03
International lunar months ordinal numbering equivalents
after leap 13-month years
ILC Hebrew Chinese Buddhist
Biblical Civil Modern Earthly Civil Traditional
L01 12 06 13 02 11 02 03 04
L02 13 07 01 03 12 03 04 05
L03 01 08 02 04 13 04 05 06
L04 02 09 03 05 01 05 06 07
L05 03 10 04 06 02 06 07 08
L06 04 11 05 07 03 07 08 09
L07 05 12 06 08 04 08 09 10
L08 06 13 07 09 05 09 10 11
L09 07 01 08 10 06 10 11 12
L10 08 02 09 11 07 11 12 01
L11 09 03 10 12 08 12 01 02
L12 10 04 11 01 09 01 02 03


The various Muslim or Islamic Hijri calendars in current use always have 12 months per year, although previous local calendars did have intercalation (Nasi’). In the Tabular Islamic calendar, there are alternating 30:29 days in a month, and the final, twelfth month has 30 days in a leap year which happens 11 times in each 30-year leap cycle (by one of several established patterns). Their mapping to L01 through L12 or L13 therefore differs every year from the previous one.

The Jewish or Hebrew year is more closely related to the concept of the ILC except that both its month numbering variants are different, as is the point of astronomic reference (i.e. Jerusalem). Ecclesiastical Jewish counting begins with Nisan (L03–L05), while civil Israeli counting starts with Tishrei (L09–L11). The month lengths are fixed except that one day may be shifted from Kislev to Cheshvan to comply with certain holiday restrictions.

The lunar calendar implied by the Christian Easter Computus is similar in that its Easter month almost always matches Hebrew Nisan.

International lunar months with Hebrew equivalents
ILC Short, common year Long, leap year Early common year Early leap year
L01 Shevat Tevet
L02 Adar Adar I Shevat
L03 Nis(s)an (Aviv) Adar II Adar Adar I
L04 Iyar (Ziv) Nisan Adar II
L05 Sivan Iyar Nisan
L06 Tammuz Sivan Iyar
L07 Av Tammuz Sivan
L08 Elul Av Tammuz
L09 Tishrei (Eitanim) Elul Av
L10 H/Cheshvan (Bul) Tishrei Elul
L11 K/Chislev Cheshvan Tishrei
L12 Tevet Kislev Cheshvan
L13 Tevet Kislev

The traditional Chinese lunisolar calendar also has the full moon in the middle of each month. Its year usually starts with the second new moon after the Northern winter solstice. Each month can be repeated as a leap month and has either 29 or 30 days. The traditional division of a month into three week-like periods, called 旬 (xún, jun, sun), of 10ish days each is not supported. The Chinese solar terms or – incorrectly – “fortnights”, i.e. half-Zodiacs lasting a 15° portion of the ecliptic, which is 15 or 16 days, are not supported (yet) either.

International lunar months with Chinese 月 yuè equivalents in common years
ILC Modern Phenological Earthly branch
L01 臘/腊 冰 ‘ice’ 🐮🐂 ‘ox, bull’, 🐄 ‘cow’, 🐃 ‘(water) buffalo’ – ‘cattle’
L02 陬 ‘corner’ 🐯🐅 ‘tiger’, 🐆 ‘leopard’ / 🐺 ‘wolf’ – ‘predator’
L03 杏 ‘apricot’ 🐰🐇 ‘rabbit, hare’ / 🐱🐈 ‘cat’ – ‘pet’
L04 桃 ‘peach’ 🐲🐉 ‘dragon’ / 🐊 ‘crocodile’ / 🐟 ‘fish’ / 🐳🐋 ‘whale’ / 🦈 ‘shark’ / 🦛 ‘hippopotamus’ / 🐌 ‘snail’ / 🦅 ‘eagle’ – ‘aquatic/avian beast’
L05 梅 ‘plum’ 🐍 ‘snake, serpent’
L06 榴 ‘pomegranate’ 🐴🐎 ‘horse’
L07 荷 ‘lotus’ 🐐 ‘goat’, 🐑🐏 ‘sheep, ram’ – ‘wooly’
L08 蘭/兰 ‘orchid’ 🐵🐒 ‘monkey’ / 🐢 ‘turtle’ / 🦔 ‘hedgehog’
L09 桂 ‘osmanthus’ 🐔🐓 ‘rooster, chicken, hen’ / 🐦 ‘bird‘ – ‘poultry’
L10 菊 ‘chrysanthemum’ 🐶🐕 ‘dog’
L11 露 ‘dew’ 🐷🐖 ‘pig, hog’, 🐗 ‘boar’ / 🐘 ‘elephant’ / 🫎 ‘deer’ / 🐢 ‘tortoise’ – ‘game’
L12 冬 ‘winter’ /
葭 ‘reed’
🐭🐀 ‘rat’, 🐁 ‘mouse’ / 🐫🐪 ‘camel’ – ‘pest’

The Buddhist calendar variants also have lunar months. While modern counting starts with the month equivalent to L04 (after the Northern vernal equinox) in common years, most traditions start the month count with L12, but some also with L11 or L10. There is no dedicated leap month, instead the month equivalent to L07 is repeated traditionally and always has 30 days. The preceding month, equivalent to L06, may be lengthened from 29 to 30 days.

International lunar months with Buddhist equivalents in common years
ILC Common year Leap year
Pali Sanskrit Burmese Khmer Lao Sinhala Thai Name
L01 Phussa Pausha, Pauṣa ပြာသို បុស្ស ປຸສສ දුරුතු ปุสส Phussa
L02 Māgha Māgha တပို့တွဲ មាឃ ມາດ නවම් มาฆ Māgha
L03 Phagguṇa Phālguna တပေါင်း ផល្គុន ຜັກຄຸນ මැදින් ผัคคุณ Phagguṇa
L04 Citta Chaitra, Caitra တန်ခူး ចេត្រ ຈິຕ බක් จิตร Citta
L05 Vesākha Vaisakha, Vaiśākha ကဆုန် ពិសាខ ວິສາຂະ වෙසක් วิสาข Vesākha
L06 Jeṭṭha Jyeshta, Jyaiṣṭha နယုန် ជេស្ឋ ເຊດ පොසොන් เชษฐ Jeṭṭha
L07 Āsāḷha Aashaadha, Āṣāḍha ဝါဆို អាសាឍ ອາສາລະຫະ ඇසළ อาสาฬห Āsāḷha 1
L08 Sāvaṇa Shraavana, Śrāvaṇa ဝါခေါင် ស្រាពណ៍ ສາວະນະ නිකිණි สาวน Āsāḷha 2
L09 Poṭṭhapāda Bhaadra, Bhādrapada / Proṣṭhapāda တော်သလင်း ភទ្របទ ພັດທະຣະບົດ බිනර ภัทรบท Sāvaṇa
L10 Assayuja Ashvin, Āśvina သီတင်းကျွတ် អស្សុជ ອັດສະວະຍຸດ වප් อัศวยุช Poṭṭhapāda
L11 Kattika Kārtika တန်ဆောင်မုန်း កត្តិក ກັດຕິກາ ඉල් กัตติกา Assayuja
L12 Māgasira Agrahayana / Mārgaśirṣa နတ်တော် មិគសិរ ມິຄະສິນ උඳුවප් มิคสิร Kattika
L13 Māgasira
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