Are there some text (specially tantras, agamas, nigamas, others) which explain how to count tithi in a different way to Vedic Jyotish?
Traditionally, fortnight start day after purnima (full moon) or amavasya (new moon). So certain dates as Vasant Panchami, should fall on day 5 after amavasya (it would be, amavasya, then day 1, then day 2, day 3, day 4, panchami), or ekadasi should fall on day 11 after full or new moon (purnima, then day 1, day 2, day 3, 4,5,6,7,8,9,10, ekadasi).
But once, I heard that, the counting should start from purnima (or amavasya) itself, so according to that, Vasant Panchami should be: amavasya, then day 2, day 3, 4, panchami (1 day before than traditional). And same with ekadasi of course.
Is there some scripture explaining this?
For example ISKCON has a very profound way to calculate ekadasi, because fasting on dashami is considered very very inauspicious, and they have a lot of regulation as to how the proper day should be calculated . But they count in the traditional way. Counting in this yogic way I'm asking, ekadashi would fall on ISKCON's dashami.