Lunar calendar or crescent moons and others employing detailed calculations. Since each lunation is approximately days (29 days, 12 hours, 44 minutes, 3 seconds, or days), it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of twelve such lunations, a lunar year, is only 354 days, 8 hours, 48 minutes, 34 seconds ( days), purely lunar calendars lose around 11 days per year relative to the Gregorian calendar. In purely lunar calendars like the Islamic calendar, the lack of intercalation causes the lunar months to cycle through all the seasons of
Karkaṭa followed by the solar month of Siṃha. The Karkata month is called "Adi" in the Tamil Hindu calendar. The ancient and medieval era Sanskrit texts of India vary in their calculations about the duration of Karkata, just like they do with other months. For example, the "Surya Siddhanta" calculates the duration of Karkata to be 31 days, 11 hours, 24 minutes and 24 seconds. In contrast, the "Arya Siddhanta" calculates the duration of the Karkata month to be 31 days, 11 hours, 13 minutes and 36 seconds. The Indian solar month names are significant in epigraphical studies of South Asia.