Two kinds of chronological systems have been used in India by the Hindus from antiquity. The first requires the years to be reckoned from some historical event. The second starts the reckoning from the position of some heavenly body.
Reckonings dates from a historical event has varied from time to time and in different parts of India. The most recent historical event commonly used in Gujarat, some part of south and the surrounding area is the Vikrama era (58 B.C.). The Vikrama era is founded after a victory of King Vikramaditya over the Saka. The years reckoned in this era are generally indicated with word Vikramasamvat or simply Samvat. They are elapsed years. In the north the custom is to begin each year with Chaitra (March - April) and each month with full moon. But in some parts of south and in Gujarat the year begins with Karttik (October - November) and the months with the new moon. The Saka, or Salivahana era (78 A.D.) is still used in some parts of India.
Reckoning time from heavenly bodies includes the sun, moon, planets and stars. While the Republic of India has adopted the solar calendar for its secular life, its Hindu religious life continues to be governed by the traditional Hindu calendar. This calendar, based primarily on the lunar revolutions, is adapted to solar reckoning. The lunar month which equals about 29 1/2 days is the period from one new moon to the next new moon. It is the time that the moon takes to complete one revolution around the earth in relation to the sun. This lunar month is further divided into about two weeks of light (SOOD) and about two weeks of darkness (VAD). This lunar month makes the year shorter than the solar year by about 11 days, and therefore one extra month is added every 30 months to correct the difference between the solar year of 365 days and the lunar year of 354 days. This year is called the lunar leap year.
While the solar system has extreme importance for astrology, which, it is claimed, govern a person's life as an individual or part of the social system, the sacred time continues to be reckoned by the lunar day
(tithi), a 30th part of the lunar month, remains the basic unit. Thus, the lunar month is only about 291/2 solar days. The tithi does not coincide with the natural day (ahoratra). The convention is that, that tithi is in force for the natural day that happened to occur at the dawn of that day. Therefore, a tithi beginning after dawn one day and expiring before dawn the next day is eliminated, not being counted in that month, and there is a break in the day sequence. The names of the NAKSATRAS to which correspond the tithis in the month lunar cycle and segments of the month in the annual solar cycle, are derived from the constellations on the horizon at that time and the names of the months are accordingly.