Few Pancha-Gauda Brahmins have month from next day of Purnima (day) to Purnima (day), that is Purnima is last 29/30 days (Purnimanta). Pancha-Dravida have month from next day of Amavasya to Amavasya . Amavasya is last 29/30 days (Amanta)Śhukla paksha is called as the bright half as the Moon changes from New Moon to Full Moon while in Krishna paksha it changes from Full Moon to New Moon. Hence it is seen that same Amavasya has same festival all over the country. Ujjain, Allahabad, Orissa, Bihar Brahmins are one few Pancha-Gauda Brahmins have month from 1 day after Purnima (day) to Purnima (day)(Purnimanta), While the people of Karnataka, Maharashtra, Goa, Gujarat, Kerala and Andhra Pradesh the Pancha-Dravida have month from 1 day after Amavasya to Amavasya. Amavasya is last 29/30 days (Amanta). Since Kanchipuram Mutt where the Adi Shankara lived and all the Pancha-Gauda and Pancha-Dravida use to visit hence Tamil Nadu developed a mixture of Panchangam and saka calendar. Similarly the place where Pancha-Gauda and Pancha-Dravida are living together as Rajasthan, Madhya Pradesh, Southern Uttar Pradesh and Chhattisgarh also show the mixtures. Also the people following Jainism follow Pancha-Dravida calendar, Amavasya is last 29/30 days.[http://www.melbournejainsangh.org/activity-tithi-calendar/