The two calendars most widely used in India today are the Vikrama calendar followed in Western and Northern India, and the Shalivahana or Saka calendar which is followed in Andhra Pradesh, Karnataka, Maharashtra and Goa.
In the year 56 BCE, Vikrama Samvat era was founded by the emperor Vikramaditya of Ujjain following his victory over the Sakas. Later, in a similar fashion, Satavahana king Gautamiputra Satakarni initiated the Saka era to celebrate his victory against the Sakas in the year 78 CE.
The Vikrama calendar begins with the month of Baiśākha or Vaiśākha (April). The Shalivahana calendar begins with the month of Chaitra (March) and the Ugadi/Gudi Padwa festivals mark the new year. Each month in the Shalivahana calendar begins with the ‘shukla paksha’ and is followed by the ‘krishna paksha’, the opposite obtains in the Vikrama calendar.
Both these calendars are based on the concept of lunar year. The lunar year is divided into 12 months and make up the six seasons. Since the calendar is based on the phases of the moon, the twelve months take 354 days, 8 hours and 34.28 seconds. This creates a difference of 10 days, 21 hours and 35.16 seconds from the actual solar year (365 days, 6 hours, 9.54 seconds). When the accumulated difference exceeds 29 days, 12 hours, 44 minutes and 2.865 seconds, an adjustment is made with an extra month (Adhika Maas), which carries the name of the previous or the next month, depending on the proximity of the month. Normally, seven extra months occur in 19 years. This calendar is mentioned in Anuwalk 6, Sookta 25 and mantra 8 of Rigvedah Sanhita. And thus has been in practice for at least 6000 years before Buddha. The mantra also mentions Adhika Maas.