The Hindu calendar, ordered by the Panchanga, is lunisolar: its months follow the moon while the year is kept in step with the sun by an added intercalary month roughly every three years, giving years of about 354 to 384 days. Its reckoning varies by region and era, the Vikram Samvat, counted from 57 BCE, and the Shaka era, from 78 CE, are the most common. The Panchanga, meaning the five limbs, is the traditional almanac that gives for each day its five essentials: the lunar day (tithi), the weekday, the lunar mansion (nakshatra), the Sun-Moon combination (yoga), and the half-day (karana). From these an astrologer determines auspicious times and the dates of festivals. These are the features of the Hindu calendar tradition, which regulates the vast cycle of religious observance across the Indian subcontinent.
Where the added month falls, and whether a lunar day is skipped or doubled, is governed by the rule that each of the twelve solar months, marked by the sun's entry into a zodiacal sign (samkranti), should ordinarily contain one lunar month; a lunar month that contains no samkranti is intercalary and termed adhika, while, more rarely, a solar month spanning two lunar new moons causes a lunar month to be omitted, termed kshaya. The two principal eras in civil and religious use differ in origin: the Vikram Samvat, beginning 57 BCE, is traditionally associated with King Vikramaditya of Ujjain, while the Shaka era, beginning 78 CE, is linked to the Kushan or Satavahana rulers and was adopted as India's official civil calendar alongside the Gregorian calendar in 1957.