<![CDATA[This study compares the Muhammadiyah Single Global Hijri Calendar (KHGT) and Turkish Diyanet, two global calendar systems that both refer to the 2016 Istanbul International Hijri Calendar Unification Congress but result in differing date determinations in certain cases. The study employs a qualitative approach using a comparative-documentary method to analyze official documents from both institutions, crescent visibility maps, and institutional correspondence between Muhammadiyah and Diyanet. Of the 131 comparable month-start dates within the range of 1447–1457 H, 129 cases (98.5%) yielded identical dates, while discrepancies occurred in 2 cases (1.5%), namely Ramadan 1447 H and Rabi’ al-Awwal 1455 H. The analysis indicates that these discrepancies do not stem from differences in astronomical threshold criteria but rather from differences in the type of crescent altitude parameters used: KHGT employs geocentric parameters consistent with the tradition of objective-universal hisab, whereas the operational implementation of Diyanet's visibility criteria is interpreted in this study as being consistent with a topocentric approach that reflects the reality of crescent visibility on the Earth's surface.. This difference shifts the position of the “first visibility” boundary curve on the Diyanet map toward the west, causing the first point where the parameters are met to fall at a different location than in the KHGT and having a decisive impact on date determination in cases involving astronomical boundaries. This finding indicates that the discrepancy between the two calendars ultimately points to a question that has not been explicitly addressed in the KHGT document: whether the 5°/8° parameter is intended as an objective geocentric astronomical condition, or as an analogy to imkanurukyat, which, in the author's interpretation, astronomically requires a topocentric approach.]]>
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