Moon calendar - Tithi
A Tithi (also spelled Thithi) is a lunar day, and it is one of the most important aspects of the Indian Almanac or the Panchang. Most Hindu festivals and ceremonies are based on the Tithi calendar, and the lunar month contains 30 days.
If you notice the difference in the date of Poornima and Amavasya, this is because the lunar calendar day begins with the sunrise. Therefore, if Purnima or Amavasya falls before sunrise is taken the previous day.