Determination of the beginning of the month among Muslims is still a polemic, there are those based on the momentum of conjunction and visibility of the new moon and there are those based on the momentum of conjunction and the cycle of days and dates in IDL (international date line).
For those who calculate the calendar based on the visibility of the hilal, some of the opinions of scholars in this issue are as follows:
If the hilal has been seen in one country, it is obligatory for all Muslims who live in other countries to fast.
Every country sees the new moon in its place. This is the opinion of the majority of Syafi'iyyah scholars.
It is almost the same as the second opinion, namely a country with a distance apart must see the new moon in their respective places, not for neighboring countries.
Therefore there are many types of Hijri calendars in the Muslim world according to the method of determining "Mathla hilal" as the date line of the calendar.
For the Islamic moon phase calendar in principle the determination of "mathla hilal" refers to the narrative of Hadith "quraib" which divides it according to the existing timezone, which is 24 timezones in the world.
With the date line moving between 24 timezone, a maximum of 4 calendar variants occur each month which divide which timezone is visible to the hilal and which is invisible to the hilal.
As for the Hijri calendar which is based on conjunction momentum and daily cycles and dates on the IDL there is no calendar variant but only one calendar applies to the world, but in this case the calendar does not follow the visibility of the moon only it still follows the conjunction cycle.