In India, Hindus follow a different calendar than what is used in the Western world. But what's the reasoning for this?
The Hindu calendar goes by many names:
Lunisolar calendar or Vedic calendar or Panchanga.
This calendar differs significantly from the western calendar, or the Gregorian calendar.
The Gregorian calendar is based on the movement of the Earth around the sun. This is the version that's used predominantly globally.
The Vedic calendar is based on both the sun as well as the moon. It uses a solar year, but divides it into 12 lunar months.
The Hindu calendar was developed in order to properly determine the right time for the performance of vratas (vows) and the celebration of utsavas (feasts). The astronomical times help us find the proper times for religious rituals, upanayana (initiation), and marriage.
Shubh Muhurat is the auspicious moment when all the auspicious work is recommended to be done. Those who reference the Shubh Muhurat before scheduling an important event, do so to receive positive energy influenced by the planets and constellations.
The lunar calendar is consistently around 10 days short of the Gregorian calendar each year, and this is made up by inserting an additional month every third year.
(The adjustment assures that the seasons, festivals, etc. retain their general position to the solar year. This is why, for example, Diwali always falls between late October and mid-November. )
The differences don’t stop there:
There are 6 seasons in the lunar calendar The Vedic calendar starts near the end of the Gregorian month of March (to coincide with new sowing season) Days are broken into different segments of time
When India declared independence, there were over 30 different variations of the Hindu calendar in order to correspond with different priests' timings for ceremonies.
The Indian government created a branch to unify all of these into the Reformed Indian Calendar.