Indian Calendar (Panchang), Variations On Dates According To Religion And Geographical Differences

The word 'calendar' reminds us basically about the solar calendar starts from January 1st. This calendar is called Gregorian Calendar now used as a universal calendar to celebrate birthdays, festivals, and historical events. But regional festivals depend on according to the dates on their own calendars, however, the system of dating may same for all calendars but the starting date of any particular calendrer is celebrated as the 1st day of each year.  So dates and years vary for each calendar. 

The Punchanga (Indian Calendars) use mostly 4 types of calendars, sake (salivahana), Vikram samvat, Hijri, and gregorian in Hindi speaking areas while the regional calendar for the particular region i.e in Bengal a day is referenced according to the Bengali calendar. and so on...there are variations on date calculations and new year day but most of the Indian calendars are a part of the national calendar and are revised as per the regional culture and language. 
The units of all calendar systems : 
1 hour = 60 minutes = 1ghatika
1 day = 24 hours (but in Indian calendars, it varies according to the time of sunrise. So a day has either less than or more than 24 hours).
1 week  = 7 days (Names of these seven days are according to the names of the nine planets revolving around the Sun).
1 month  = 30 days but varies to 28, 29, 30,31, or 32 days according to the lunar cycles.
1Year = 12 months.

There are mainly three-date counting systems;
1. Solar (According to the position of the Sun relative to the stars)     
2. Lunar  (According to the position of the moon relative to the Earth )
3. Lunisolar (based on the moon cycles and position of the Sun relative to the stars and Earth)

Solar and Lunisolar calendars used in India

1. Indian National Calendar (Saka Calendar ).

2. Gregorian calendar:  Used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the 'tropical' or 'solar' year that the inaccuracy had caused during the intervening centuries. 

Months of Gregorian Calendar
No.	Name	Length in days
1	January	31
2	February	28 (29 in leap years)
3	March	31
4	April	30
5	May	31
6	June	30
7	July	31
8	August	31
9	September	30
10	October	31
11	November	30
12	December	31

Day	Name	Indian name
1	Monday	somavar(सोमवार)
2	Tuesday	Mangalvar(मंगलवार)
3	Wednesday	Budhvar(बुधवार)
4	Thursday	Guruvar (गुरुवार)
5	Friday	शुक्रवार(shukravar)
6	Saturday	शनिवार (shanivar)
7	Sunday	रविवार (ravivar)

3. Bengali Calendar: Used as the official calendar among Bengali-speaking peoples, in Bangladesh and in West Bengal and Tripura states of India. it is the revised version of the sake Calendar, But 1st month of the year is Vaiśākha while in the sake calendar Chaitra is the starting month.

4. Malayalam Calendar: Used in the Kerala state of India, started from 825 CE.

5. Oriya Calendar: In Odisha state of India, Lunisolar, the new year day is called 'Pana Sankranti'.

6. Assamese Calendar: It is the revised calendar of the sake calendar and the year starts from the month of Vaisakha (Bohag).  The New Year in the Assamese calendar is known as Bohag Bihu. The calendar is counted from the date of the ascension of Kumar Bhashkar Barman to the throne of Kamrup. It differs 593 years from the Gregorian calendar.   

Assamese Name	Romanization	Sanshkrit Name	No. of Days
ব’হাগ	Bohag	Vaiśākha	31
জেঠ	Zeth	Jyeṣṭha	31
আহাৰ	Ahar	Āṣāḍha	32
শাওণ	Xaun	Śrāvaṇa	31
ভাদ	Bhado	Bhādrapada	31
আহিন	Ahin	Aśvina	31
কাতি	Kati	Kārtika	30
আঘোণ	Aghun	Mārgaśīrṣa/Agrahayana	29
পুহ	Puh	Pauśa	29
মাঘ	Magh	Māgha	30
ফাগুন	Fagun	Phālguna	30
চ’ত	Sot	Caitra	30

7. Tullu Calendar: Traditionally used in Northern Kerala.

8. Nanakshahi Calendar: 12 months, used in Sikhism based on the teachings of Sikh Gurus.