The modern western calendar that we are accustomed to is based on the sun in which a year (365 days, 5 hours, 48 minutes, and 46 seconds) is the time required for the earth to complete one orbit around the sun. This solar year is composed of 12 arbitrarily assigned months which have either 30 or 31 days, with the exception of February.
The Indian calendar is based on both the sun and the moon. The Indian calendar uses the solar year but divides it into 12 lunar months. They are listed in order from beginning to end: Kãrtik, Mãghshar, Posh, Mãgh (Mahã), Fãlgun, Chaitra, Vaishãk, Jeth, Ashãdh, Shrãvan, Bhãdarvo, and Ãso.
A lunar month is the time required for the moon to orbit once around the earth and pass through its complete cycle of phases. These months are formulated not arbitrarily, but in accordance with the successive entrances of the sun into the 12 rãshis, the 12 constellations of the zodiac marking the path of the sun.
A lunar month is precisely 29 days, 12 hours, 44 minutes, and 3 seconds long. Twelve such months make up a lunar year of 354 days, 8 hours, 48 minutes, and 36 seconds. To ensure that the corresponding seasons according to the lunar months coincide with those of the solar year, an extra month is inserted every 30 months (approximately every 2½ years) because 62 lunar months are equal to 60 solar months.
As a result of the adjustment, the seasons and festivals retain their general position relative to the solar year.Each lunar month is divided into two pakshas (two parts) – the sud or shukla paksh (the bright half of the month when the moon waxes from a new moon to a full moon) and the vad or krishna paksh (the dark half of the month when the moon wanes from a full moon to a new moon).
Each paksha is divided into 15 tithis (lunar days) which follow the names of Sanskrit numerical system. The era that is currently used in the Indian calendar is the Vikram Samvat Era, which began in 57 BCE when King Vikram drove off a Greek invasion of the Malwa region and came to the throne. Thus, we have the following conversion to the Indian year.
If the western calendar date falls between Kartik sud 1 (the beginning of the Indian Year) and December 31st (the end of the western calendar year), then 57 years should be subtracted from the Indian year to make the conversion. If the western calendar date falls between January 1st (the beginning of the western year) and Aso vad 30 (the end of the Indian year), then only 56 years should be subtracted to make the conversion.