1.          Ancient Era: From the world’s creation to the fall of the Roman Empire.

      2.          Medieval Era: From the fall of Rome to the conquest of Constantinople by Sultan Mehmet II.

      3.          Modern Era: From the conquest of Constantinople onward.

Events are often dated relative to significant milestones, such as the creation of Adam, Noah’s flood, Jesus’ birth, Vikramaditya’s era, or Prophet Muhammad’s migration (Hijra). Today, the Christian and Islamic (Hijri) calendars are most widely used.