It is said that the beginning of Ramadan and Eid are determined by checking whether the Moon can be seen. However, this only applies for the city of Mecca, not outside it.
The centre of the Earth is Mecca, Saudi Arabia. When the Moon appears above the centre of the Earth, over the Kaaba, that is the correct time to begin Ramadan.
Our time zones used to be measured according to Greenwich Mean Time (in London, UK). For example, we would say that a time zone is four hours ahead of GMT, five hours behind it, etc. However, now we use UTC to measure time.
Similarly, once the Moon is sighted in Mecca above the Kaaba, that is when you start fasting. If you determine your fast according to this, you will never miss out on important days like Laila Tul Qadr or Eid. If you look at the Moon in the areas where the time zones are ahead of Mecca, that is wrong. No matter where you live, you must go according to whether the Moon is sighted in Mecca.