Hijri Calender is based on the moon cycle so the moon will be seen on the different time on different geographical location so you must has to follow the calender of your location.
Can we follow same time all over the world. can the 12 pm in India will be equal to 12 pm of America, Not na . Since time is calculated on sun you can't consider it as same on the same geo area.
And this is Not only in Islamic Calender the date difference is also in English calender.
Allah's Messenger (peace_be_upon_him) said: Whenever you sight the new moon (of the month of Ramadan) observe fast, and when you sight it (the new moon of Shawwal) break it, and if the sky is cloudy for you, then observe fast for thirty days.
Allah's Messenger (peace_be_upon_him) said: Observe fast on sighting it (the new moon) and break it on sighting it. But if (due to clouds) the actual position of the month is concealed from you, you should then count thirty (days).
Irrespective of geographical location.