Hindu Vrat Tyauhaar Science of date change due to lunar Date

Purnima and Amavasya date from Pratipada:-

Dates are counted from the Pratipada of Shukla paksha. The full moon is called 15 and Amavasya is called 30. The day when there is a difference (distance) of 180° between the Sun and the Moon, that is, the Sun and the Moon are faces to face, then it is called a full moon date. And when the sun and the moon are at the same place i.e. there is a difference of 0º then it is called Amavasya Tithi. The total value of Bhachakra is 360º, then one Tithi = 360 » 30 = 12º i.e. a Tithi is formed when there is a difference of 12º between the Sun and Moon.

Growth of date and decay of date in Indian astrology:-

For example, Pratipada (Shukla Paksha) from 0º to 12º, second from 12º to 24º, and Amavasya from 330º to 360º, etc. In the tradition of Indian astrology, there is the growth of a date and decay of a date. The date on which sunrise occurs twice is called Vridhi Tithi and in which there is no sunrise, then that date gets destroyed. For example, a Tithi starts before sunrise and continues for the whole day till 2 hours after sunrise, then this Tithi touches two sunrises. That’s why this date increases. Similarly, another date starts after sunrise and ends before sunrise on the second day, because this date does not touch a single sunrise, so due to decay, the date is called decay.