The determination of the day in Islam is particularly influential, especially in terms of worship, the determination of the beginning of the islamic lunar month, which corresponds to the circulation of the Moon. One Hijri month comprises 29 or 30 days. There are two methods in calculating the beginning of the islamic lunar month, that is the hisab method and the rukyat method. From both approaches, the term mathla' (area boundaries based on the reach of the new Moon) originates. Mathla' is divided into mathla' ikhtilaf (local mathla') and mathla' ittihad (global mathla'). From the two mathla', there are diverse opinions regarding the use of mathla'. Therefore, the government is present and seeks to avoid this imbalance through imkan ar rukyat. This research involves qualitative research, categorized as library research, described descriptively with systematic, correct explanations and relevant sources. The dialogue on the two mathla' is reaping the pros and downsides among numerous madhhab imams and Muslim personalities. The three madhhab priests believe that global mathla' is the reference for the people, except for the Shafi'i priest. Figures such as Hasbi Ash shiddieqy consider that global mathla 'as a basis for attaining the unity of the ummah. In contrast to Thomas Djamaluddin, Basit Wahid and Yusuf al-Qaradhawi who prefer local mathla' as a reference. This study also examines the application connected to mathla', namely the Islamic Calendar at Turkey's International Hijri Calendar Unity Conference (IHCUC) congress. What has implications or effects if the new moon on one continent has appeared is not necessarily the same on other continents. This happens because  the geographical position of the area is different. It gives a favourable critique to establish a shared opinion for the benefit of the people.