The Unified Global Hijri Calendar (UGHC) principle of global synchronization ensures that the beginning of a new month follows a single, unified date worldwide similar to the Gregorian calendar system. There are several principles to ensure the calendar maintains consistency and accuracy while aligning with astronomical and global considerations.
The principle of global synchronization of day and date worldwide
The synchronization of day and date ensures that the start of a new lunar month follows a single, unified date across the world—mirroring the Gregorian calendar system, where one day corresponds to one date worldwide. To achieve this global consistency requires avoiding two key challenges: Firstly, by dividing the Earth into separate date zones, which would create discrepancies in determining lunar months across different regions. Secondly, by creating new lunar date lines, such as the Lunar Date Line proposed by some Islamic calendar scholars. Adding additional date lines would complicate the calendar system rather than unify it. The only way to maintain this alignment is to adopt the International Date Line, which is globally recognized and accepted by the international community.
The use of astronomical calculation (hisab)
In determining the beginning of an Islamic lunar month, astronomical calculations hold the same status as rukyat (moon sighting), as outlined in [Tarjih Decision XXVI, 1424 H/2003 M]. Thus, the use of astronomical calculation is considered valid and in accordance with the Sunnah of Prophet Muhammad (peace be upon him). Both moonsighting and astronomical calculation serve as tools to determine the start of the Hijri month. However, astronomical calculation is regarded as a more precise method, providing certainty in determining the start of the month. This is why astronomical calculation should be prioritized over moonsighting. From a technical calendar perspective, moonsighting poses significant challenges to achieving global unity. In fact, creating a calendar solely based on moonsighting is impossible. This is because moonsighting can only confirm the start of a new month one day in advance (D-1). On the other hand, a calendar must be able to accurately predict dates far into the future and provide at least a year’s worth of scheduled dates. The Second Expert Meeting, held in Rabath, Morocco (1429 H/2008 M), concluded that resolving the challenges of the Islamic calendar is only possible by accepting hisab as the primary method for determining the start of the lunar month—just as hisab is already used in determining prayer times.
Unity of Matlak
The unity of matlak refers to the concept that the entire Earth’s surface is considered as a single calendar zone. Consequently, the idea of matlak diversity or ikhtilāf al-maṭāliʻ becomes impossible to adopt. A zonal calendar divides the Earth’s surface into multiple date zones or distinct matlak regions. As a result, it is impossible to synchronize dates to fall on the same day worldwide. In the context of UGHC (Unified Global Hijri Calendar), there is only one calendar zone or matlak, encompassing the entire Earth’s surface. Ibn ‘Āsyūr (d. 1393/1973) affirms this principle:
The proofs from the Sunnah and the opinions of the four major schools of Islamic jurisprudence align with the principle of not considering differences in matlak (moon sighting regions). Hanafi scholars state: “This is the opinion of most mashayikh (scholars).” Maliki scholars declare: “This is the well-known opinion.” Shafi‘i scholars mention: “On this issue [within the Shafi‘i school], there are two recognized opinions.” Hanbali scholars affirm: “There is no disagreement that the sighting of the moon in one region applies to all other regions.”
Global implementation of calendar parameters at a single location
Global implementation of calendar parameters, established at a single location is sufficient. This concept entails applying the calendar parameters (Moon altitude of 5⁰ + elongation of 8⁰, including corrections) globally across the entire world once they are met at any specific location on Earth. The 5⁰ + 8⁰ parameter is analogous to the crescent visibility criterion (imkanu rukyat), as under such conditions, the crescent moon becomes theoretically visible. In fact, there are imkanu rukyat parameters lower than this, such as the 3⁰ + 6.4⁰ criterion. However, it is essential to note that crescent visibility (imkanu rukyat) in this context differs significantly from the common understanding, where imkanu rukyat is determined at a specific location at sunset. That traditional view reflects a local concept, while in the UGHC (Unified Global Hijri Calendar) system, the 5⁰ + 8⁰ parameter—analogous to imkanu rukyat—is applied globally. This means that it must be met somewhere on Earth before 00:00 UTC. If the condition is met after 00:00 UTC, the new lunar month still begins, provided that the easternmost time zone has experienced ijtima‘ (conjunction) before dawn and the 5⁰ + 8⁰ parameter has reached the continental America. Furthermore, the emphasis of this parameter is not simply on whether imkanu rukyat has been achieved but rather on ensuring two key principles: First, the easternmost time zone should not be forced into the new lunar month if ijtima‘ has not occurred before dawn in that region. Second, the westernmost time zone should not be required to delay the new month if the crescent is already clearly visible on their horizon.
The permissibility of globally applying calendar parameters is rooted in the general meaning of the hadith: ṣūmū li ru’yatihi wa afṭirū li ru’yatihi (fast when you see [the crescent] and break your fast when you see [the crescent]).[3] According to this broad interpretation, all Muslims are obligated to fast once a crescent sighting (rukyat), including the possibility of sighting (imkanu rukyat), occurs—without restricting its validity to a specific location. Thus, wherever on Earth rukyat and calendar parameters are fulfilled, fasting becomes obligatory for the entire Muslim community. This principle negates any difference in matlak, considering the entire world as a single matlak. Al-Haskafi (d. 1088/1677) states: “Differences in matlak… are not considered… This is the position held by most Hanafi jurists, and it is the ruling issued in fatwas. As a result, those in the East must fast based on the sighting of those in the West.” Ibn ‘Abidin (d. 1252/1836) affirms: “This is the position upheld in the Hanafi, Maliki, and Hanbali schools, based on the generality of moon sighting in the hadith: ‘Fast when you see [the crescent].’” AlNawawi notes: “Some of our scholars state that a sighting in one location applies to all inhabitants of the Earth.”
Acceptance of the International Date Line (IDL)
The entire global community, including Muslims, has accepted the currently established International Date Line (IDL) as the boundary separating two consecutive days or dates. This line is positioned along the 180° meridian of longitude, marking the point where a new day begins. Muslims determine Friday—a day with obligatory Jumu‘ah prayers—based on this boundary. As a result, there is no possibility of creating an alternative date line or relocating it elsewhere, as this would lead to the dualism of days, causing confusion in determining dates and religious obligations.
