Hijri Calender
There are two different things to understand. Lunar months are appointed by God for different islamic rituals like Ramadan, Hajj, etc. but Hijri calendar, to count the years from the event of hijra, was adopted by Muslims in the era of the second caliph Umar. We can follow our local or international calendar but appointed months of Islamic rituals cannot be changed. Ramadan is a month of blessings. Difference of dates due to different climates or because of the moon hidden in clouds does not affect the matter of blessings. Muslims are obliged to start the month with whatever resources they have, the blessings of God bestow upon them accordingly.