Saudi Arabia does not allow it IN PUBLIC, but no one forbids them from worshiping their own way in private so long they do not go out and proselytize , and because there aren't any non-Muslim Saudis . All of the people who are not Muslims are basically foreigners who do not intend to stay long. Why should the Saudi govt be concerned with people who are not citizens of the country , they have their own country embassies to take care of their "expatriate" needs.
As I said in my other answer they do not allow public proselytization (religious propaganda) of people from their current faith , which also applies with in Muslim sects public proselytization from one sect to another is not allowed. That is not an issue of religious freedom , rather an issue of the government being a monarchy does not want any destabilizing or religious polarization so it can better control the country (don't blame Islam for that). Plus they never said they were a democracy to begin with.
And what makes you think you can say "they are behind 500 hundreds" , on the calendar. These are NOT THE SAME calendar , the Roman Gregorian & Islamic Hijri calenders commemorate 2 different events , and one is solar the other is lunar, they are not a ruler or a measure stick for people's progress or way of thinking. Also the difference is less than 400 years because the Christian calendar was created 2 centuries after B.C. (counted from B.C). It is not a measure because 1000 years ago most of Europe was in the dark ages while it was a scientific and knowledge boom in the Arabian region. All these sciences that Arabs & Muslims have founded are actually the foundation of every branch of science in the west today. Where do you think the numerals , the clock, the compass , civil law, the world globe, and countless other things originate from ..
Also in is interesting that you say "our" when you type for Philippines which did not have any sort of comprable calender to the Islamic calendar back than. Your country have been visited by Muslims long before the European imperialist states tried to take over your country , interestingly using knnowledge and maps charted by Arabs.How do you think the people in south of Philippines became Muslims?. It is a matter of science that the lunar calendar used by Muslims is more accurate than the solar (because the moon is the closest celestial body to earth), and there are no leap years to make up for the 1/4 day in Gregorian calendar. Know that Arabs had a the same calendar prior to Islam and it has been used for thousands of years , only the Hijri year NUMBER counts the years from the Hijra (migration) date of Prophet Muhammed.(Before that Arabs referred to lunar year by name or major event , same as Chinese convention for lunar year naming) The months name and the lunar cycle is as old as history itself , nobody even knows how old. Arabs in north Arabia also used another solar calendar which pre-dates the Gregorian (still in use in Levant countries) , it was used by the Romans until they started the count from B.C. (After the Roman emperor converted to Christianity). But for the Arabic solar calendar it probably pre-dates the Roman one by 3000 years since the Babylonian era.