The measurement of Time in India is unique.

While other calendars take Time to be Linear,that is Time flows in a straight line, yesterday,today and tomorrow,Indian cosmology takes Time as Cyclic.

It is for our convenience that we follow the Julian calendar,where midnight is reckoned as the end of one day and the beginning of another.

For Hindus,new day begins at 3.45 am approximately.

And in Hinduism Time is not Linear,but Cyclic.

That is Time is Eternal and keeps on recurring,in our Perception.

In fact Time is reckoned as synonymous with Reality.

Time is a spectacle,along with Space.

Both are the tools embedded in our thought processes and as such make us Perceive things only through them.

Try forming a sentence,or even a Thought, without reference to Time and Space.

It is impossible.

Please read my article Time Cyclic,not Linear.

I also recommend the documentary Time available in Amazon Prime Videos.

So the calculation of Time in India is different.

The Indian Calendars are of two kinds.

One is designed based on the movement of the Sun,Suryamanasa,and another Chaandramanasa,based on Moon’s movement.

In general there are five calendars being followed in India.

Vikrama,in North,

Shalivahana in Deccan,

Tamil in Tamil Nadu,

Malayalam calendar in Kerala and

Bengali calendar in Eastern India.

There are more regional versions of Calendars.

These vary.

Now ,in general,we follow the Julian Calendar in our daily life.

Here we have a confusion confounded with BC and AD.

You have two systems of dating one before Christ (BC) and another after Christ,AD.

This Calendar is linear,that is year 2000,2001,2002…….

But Indian calendar has sixty years of the same name,running again and again.

That is one year starts with Prabhava,Vibhava….,runs for sixty years and it again starts with Prabhava.

So to find the exact date one needs other references,because if you say an event took place in Prabhava, you do not know which Prabhava as Prabhava keeps on occuring once in sixty years.

But in Jukian calendar you mention 1960,1868 etc and you can locate the year.

This is because Julian Calendar takes Time as Linear.

But Calendar of Hindus differ and the names of years get repeated.

So to pinpoint a specific day or date following parameters are added in Hundu calendar.

Ayana, movement of the Sun to Cancer or to Capricorn,

Ritu,Season,

Samvatsara,

Month,

Paksha,waxing or waning of the Moon,Sukla or Krishna Paksha,

Thithi,the day from the New or Full Moon,

Nakshatra,the Star of the day.

This gives you a comprehensive view to arrive at a particular date.

(Even this elaborate system is not foolproof in fixing exact date. I wilk be writing on this.)

But to get this date or day,you need a starting point.

Vikarma,named after King Vikramaditya begins with his time,Shakivahana with his Time ,Tamil ,Malayalam and Bengali begin with Kali,that is Kaliyuga.

But there is a fundamental base year for Hindu Calendar.

It is the Saptha Rishi calendar.

Not many are aware of this.

There are, according to Hindu cosmology,Seven Seers,Rishis,who do not die at the time of the dissolution.

The post is Ex officio.

The Rishis,seven in numbet change for each yuga/Manvantara,a time scale.

Please refer my article on Manvantara.

The seven or Saptha Rishis , for Kakiyuga are,

Atri,

Bhrugu,

Kutsa,

Vaisishta,

Gautama,

Kasyapa and

Angirasa.

The Rishis differ in some puranas.

Saotha Rishi Era, Calendar begins in 6676 BC,that is 8600 years ago.

This has been mentioned by Kalhana, Indian King and Historian..Greek Historians Pliny and others recird this.

This would predate events in Indian History.

The Saptarshi cycle

A lesser-known Hindu system of time-reckoning is the Saptarshi cycle of 3600 years (possibly based on the 60-year cycle, see ch. 2.4.5. below). At any rate, by the Christian age we find writers who take this concept of a 3600-year cycle literally, and it is hard to either prove or refute that this may have been a much older tradition.

The medieval Kashmiri historian Kalhana claimed that the previous cycle had started in 3076 BC, and the present one in AD 525. J.E. Mitchiner has suggested that the beginning of the Saptarshi reckoning was one more cycle earlier, in 6676 BC: “We may conclude that the older and original version of the Era of the Seven Rsis commenced with the Seven Rsis in Krttika in 6676 BC, used a total of 28 Naksatras, and placed the start of the Kali Yuga in 3102 BC. This version was in use in northern India from at least the 4th century BC, as witnessed by the statements of Greek and Roman writers; it was also the version used by Vrddha Garga, at around the start of the Christian era.”22 This would roughly coincide with the start of the Puranic dynastic list reported by Greco-Roman authors as starting in 6776 BC.

Source.

http://www.indicethos.org/Astronomy/The%20SaptaRishi%20Cycle