Umar (may Allah be pleased with him) narrates:

The Messenger of Allah (peace and blessings be upon him) stated:

When night comes from that direction (from east), day goes to that direction (to west) and sun sets,a  fasting person can break his fast. (Bukhari, Sawm: 43; Muslim,  Siam: 51; Abu Dawud, Sawm: 19; Tirmidhi. Sawm: 12)

Time for iftar comes actually when the sun sets. The fast is broken. The fast of the next day is intended. However, today people act depending on the calculation of the calendar, the adhan for maghrib (evening) salat is called based on the calendar and time for iftaar comes.

In fact, calculation on the calendar is made based the sun. Both the time for suhur (a meal taken before fajr in the month of Ramadan) and time for iftar are determined in accordance with the location of the sun, its state and rising and setting. That hadith declares the time for iftar clearly.