There are two systems of Vedic astrology — Solar calculation and Lunar calculation. The Vaisnava devotional lineage follows the Lunar calendar to calculate the exact Tithi (the Lunar Day). There are twenty-seven constellations and they travel with the Moon. As the Moon travels, the Tithi is carried by the Moon. Therefore, the Vedic calculation of religious dates works per the position of the Moon, and NOT according to the geographical position of a particular country like some may be used to following.
For instance, on this authentic Vaisnava calendar, this year’s Gaura Purnima falls on March 12th 2017, not any other day, and that same date is to be followed everywhere in the world.
This Vaisnava calendar is calculated by Sri Krsna Balaram Swamiji, who appeared in and was raised by an enlightened Gaudiya-Brahmana family, descending from Kasyapa Muni, in the holiest land of Sri Vrindavan Dhama in India. He is expert in Vedic knowledge and holy astrology. He annually calculates the Vaisnava Calendar telling the proper Holy dates and Ekadasi dates. This calendar is calculated with exact agreement with the ancient scripture Haribhakti Vilasa of Srila Sanatana Goswami, a direct disciple of Lord Chaitanya Mahaprabhu.
[From Sri Krsna Balaram Swamiji talks, KBS0102]