Q: Could you be more specific? The Islamic Calendar?
A: Because the dates are decided according to the Islamic calendar... As for the Iftar times, it's because the length of the days change throughout the seasons. For example, tomorrow, the day wil be shorter than today. So, Iftar will come sooner. [EDIT] Well, there are 11 days less in the Islamic lunar calendar than in the solar calendar (which is what we use in today's society). This is why Ramadan comes earlier and earlier every year. I just did some research to see why exactly we have an Islamic calendar. It's actually very simple. It's to help us obey the rules of the Qur'an, and celebrate or fast on specific days that you can only know by looking at the moon (which is why the Islamic calendar is also called the lunar calendar). For example, Muslims know when it's Ramadan by looking at the moon.