Most of disputes in history starts with dates, one claiming predating other. Let us see how difficult it is to know the exact year of existence of particular person from Inscriptions. We are restricting ourselves to Inscriptions as going to literature will open another Pandora box.
Shalivahana saka
The Inscription of Pulakeshi talks synchronises the era with Kali era and uses the term Shalivahana sake. This era is also called Saka era. This is the most consistently used era and also currently used by Govt of India as Well. Historically this marks the defeat of Saka ruler of Ujjain by Shalivahana of Paithan. This is era is used extensively in Karnataka, Maharastra and Andhra. The start of era is 78AD. However Indologists have many theories for this era. But it is generally agreed that Kanishka was the first to use Saka era , so the origin is given to him. But kushana chronology itself is debated. Saka era is labled in various ways such as saka-nrpa-Kale, saka-varsesv-atitesu, Saka-varse, Salivahana sake, sake and so on. Except the earliest years Saka era dates are labeled saka year.
started by Chalukya Vikramaditya IV upon accession to throne in 1076AD. The Inscriptions of Vikramaditya mention this era ,many times the Saka era is also mentioned.
Inscriptions are dated in some many ways. Ranging from simple year date
(regnal or era ) to detailed year month tithi(lunar day), week day and/or other calendrical or astronomical dates. Let us see the Era or dating used.
(regnal or era ) to detailed year month tithi(lunar day), week day and/or other calendrical or astronomical dates. Let us see the Era or dating used.
Regnal Year
This is practice dating records from regnal years of king , this method is used in most of the inscriptions. Beginning from Ashoka , continuing with satavahana, Ikshvaku, Vakatakas continued record in regnal years in their inscriptions, this continued in medieval period with Palas, Pallavas, Cholas, Pandyas. A typical inscription with regnal year, might say “ The great king of so-and-so with so-and-so titles with so-and-so achievements donated in the first year of of his reign “. The inscription may add the tithi or day of the week, or month in addition the regnal year. Unless additional reference date or another king or ruler or event is provided the dating of the inscription is not absolute. These inscriptions have to be dated by techniques similar to paleographic records or undated inscriptions. But sometimes due to historical synchronization we can find the year and sometimes month and date of the particular inscription, but this is not the case with most inscriptions. The successive rulers of dynasty have given inscriptions ,so we can date them successfully. But sometimes some successive rulers have same names which compounds the problem. The problem is with regnal years everything is relative and fluid, with change of date of one inscription every other inscription has to be re-dated. If Greek have not come to India , we will not be able to date Ashoka and with Ashoka the entire Indian History.
Year of an Era
Beginning with First century BC some inscriptions dated the years in continuous era. In this practice the kings issuing the inscription dates the inscription in the regnal years of the previous kings instead of his accession. This system is seen in the Dynasties. Sometimes they are followed even when the dynasties have fallen and new one takes over. But this information is not absolute, the name of the era may be left out since it is a prevalent one and common knowledge. In addition the year may be given in numbers or words or both. Sometimes the year is given as chronogram(bhutasamkya), with digits expressed in words for items associated in particular order. The suggested numbers have to read in reverse order(according to the principle Ankanam Vamato gatih, numbers run leftward). Sometimes both common era and Regnal year can be found.
Let us see some of the Era's
Jain Nirvana Era
This date starts with Nirvana or salvation of Last Jain Tirthakarna Vardhamana Mahavir. While Vicarasreni(1310AD) in Merutunga(Prakrit) gives date as 470 Years to vikrama Era, Nemichandra(1084AD) in Mahaviracariam(prakrit) gives the date as 605 years and 5 Months to before the start of Saka Era. So the dispute will be there as the Mahavira date is not certain. The Sravanabelagola Inscription equating Vir- Nirvana year 2493, Vikrama 1888 and Saka 1752 is an example.
Buddhist nirvana Era
Cantonese records say each year after passing of Buddha was represented by a dot and so the date of Buddha is 486BC , the start of Buddhist nirvana Era. But there is no agreement in Buddhist world. So the dates are disputed.
Vikram Era
This Era starts in 50 BCE by King Vikramaditya. Vikramaditya regained his ancestral kingdom in Ujjain by expelling the Sakas from there after 9 years of their rule (66-57 BCE). In order to commemorate his victory over them, he introduced a new era called Vikram Samvat (or Malawa Samvat) in 57 BCE. This story is also disputed.Shalivahana saka
The Inscription of Pulakeshi talks synchronises the era with Kali era and uses the term Shalivahana sake. This era is also called Saka era. This is the most consistently used era and also currently used by Govt of India as Well. Historically this marks the defeat of Saka ruler of Ujjain by Shalivahana of Paithan. This is era is used extensively in Karnataka, Maharastra and Andhra. The start of era is 78AD. However Indologists have many theories for this era. But it is generally agreed that Kanishka was the first to use Saka era , so the origin is given to him. But kushana chronology itself is debated. Saka era is labled in various ways such as saka-nrpa-Kale, saka-varsesv-atitesu, Saka-varse, Salivahana sake, sake and so on. Except the earliest years Saka era dates are labeled saka year.
Kalachuri-Cedi Era
The
date of origin of this era is also doubtful. The date of 248AD based on
the accesion of Abhira king Isvara dutta is now not valid.
Vallabhi Era
Thisis used in Kathivad and neighbhourhood.It is synchronous with Karttikadi Vikrama year 376 (318-319AD) and Saka 242-42. Inscription in the era are available from 82 to 945. It is also said this is gupta era continued and the era corresponds to accession of Chandra gupta I. Guptas don't use the era in the earlier inscriptions. So the claim is questionable.
Most historians attribute to Accession of Chandragupta I. The dates are calculated by adding years 319 to 322 years to that is found in the inscriptions.
Thisis used in Kathivad and neighbhourhood.It is synchronous with Karttikadi Vikrama year 376 (318-319AD) and Saka 242-42. Inscription in the era are available from 82 to 945. It is also said this is gupta era continued and the era corresponds to accession of Chandra gupta I. Guptas don't use the era in the earlier inscriptions. So the claim is questionable.
Most historians attribute to Accession of Chandragupta I. The dates are calculated by adding years 319 to 322 years to that is found in the inscriptions.
Ganga (Gangeya Era)
Many inscriptions of Eastern Gangas and Eastern Kadambas in Karnataka , Andhra and Orissa. The Ganga Era is also disputed ,but it is generally agreed it starts in 475AD based on inscription of Kadamba feudatory Dharmakhedi.
Sri Harsa Era
Al-Biruni (11th century AD)suggests that Harsa Era was prevalent in the area of Mathura-Kannuaj starting with 457BC. But there seems to be no common such era in India before Saka and Vikram Era. In fact Al-Biruni learnt in Kashmir that Sri Harsa era started with Sri Harshavardhana in 606AD, the Mathura pandits seems to have tricked Al-Biruni to such date.
Bhatika Era
The historical origin of Bhatika era is disputed. It starts at 623 to 625AD. The dispute here is due to association the close synchronisation and association with Islamic Hirja Era 622AD in western India and close to regions of Arab Incursions. Absence of Early inscriptions is explained as the solar modification of Muslim era. Mr.Dasharatha sharma says it represents era of Bhatia rajput Clan. Several unpublished inscriptions near Jaisalmar have cited to indicate Bhatika samvat. However Mr. Mirsa and Mr.Sarcar argue some of the inscriptions are Harsa Era.
Kollam Era
This era starts at 824AD, used in Kerala and around Kerala mostly in Malayalam and Tamil sometimes in Sanskrit as well. The reason for the start of the Era is controversial as it marks the starts of the foundation of Kollam, but Kollam existed prior to it. Some scholars say it is related to Saptarsi era, but the geographical distance makes the theory untenable.
Bhauma Kara Era
This era started by Bhauma Kara kings of orissa. The era starts in 831AD probably of ruler Ksemankaradeva. Earlier the era was considered to be harsa era.
Nepali or Newari Era
This era was started by Jayadevamalla. The era starts in 871AD. The earliest Nelpali inscriptions date themselves to Saka or Vikrama or Gupta or Licchavi Era.
Chalukya-Vikrama Era
This era was 
started by Chalukya Vikramaditya IV upon accession to throne in 1076AD. The Inscriptions of Vikramaditya mention this era ,many times the Saka era is also mentioned.
Lakshmana Sena era
This era starts in 1117AD in Gaya region of Bihar. This era is attributed to Accession of Sena King Lakshmana sena though the dates do not match.
Bengali San
The Era is Named Bengali San(Sen) is in use in Bengal. It is solar year and runs with saka era. The Months are however Lunar. It starts at 516 Shalivahana Saka.
Vilayati Year
This is another Solar Saka year used in parts of Bengal and Chiefly in Orissa. The Months like Bengali San is Lunar. The start of the Vilayathi year is same as Bengali San 516 Shalivahana Saka. There are two differences though, First begins solar year in Kanya which is bengali sen Ashvina or Assin. Second the months begin on the day of sanskranti instead of following 2nd or 3rd day.
Amli Era
This era is used in Orissa. The Amli commenses from the birth of Indradyumna, raja of orissa on Bhadrapada Sukla 12th and each month commenses the moment the sun enters the sign. Amli san is used in business transactions and in the courts of Law in orisssa.
Fasali Year
This is harvest year introduced by Akbar,originally derived from Mohammadan year and bearing the same number, but beginning in july. It was in most parts of India Solar year, but different customs in different parts of india has made it divergent. There is Luni Solar and North west Fasali year.
Maharatta sur san or Shakur san
This is sometimes called Arabi san. It is extensively used in Mahratta domains. This is nine years behind Fasali of Deccan.
Magi San
This is used in Chittagong. It is similar to Benglai san. The only difference is it is 45 years behind.
Simha Samvat
This era is used in Kathiavad. THe era starts in 1036-37 Saka
Lakshmana Sena era
used in Tirhut and Mithila, but always with Saka or Vikrama era. Era starts in 1105-6 AD.
Ilaki Era
The Tarlkh-i-Illahi or Mighty Divine era was established by Akbar. It dates to his Accession 14 Feb 1556 (Friday 2nd Rabi-us-sani AH 963) . It is extensively used in Coins of Akbar and Jahangir, but Shah Jahan seems to discontinued.
Rajyabhisheka Saka
It is also called Mahratta Raja Saka Era. Saka here is era. This era was established by Shivaji on the day of accession (Jyeshta sukla Trayodasi (13th) of Saka 1596 expired or 1597, Ananda Samvatsara)
Bengali San
The Era is Named Bengali San(Sen) is in use in Bengal. It is solar year and runs with saka era. The Months are however Lunar. It starts at 516 Shalivahana Saka.
Vilayati Year
This is another Solar Saka year used in parts of Bengal and Chiefly in Orissa. The Months like Bengali San is Lunar. The start of the Vilayathi year is same as Bengali San 516 Shalivahana Saka. There are two differences though, First begins solar year in Kanya which is bengali sen Ashvina or Assin. Second the months begin on the day of sanskranti instead of following 2nd or 3rd day.
Amli Era
This era is used in Orissa. The Amli commenses from the birth of Indradyumna, raja of orissa on Bhadrapada Sukla 12th and each month commenses the moment the sun enters the sign. Amli san is used in business transactions and in the courts of Law in orisssa.
Fasali Year
This is harvest year introduced by Akbar,originally derived from Mohammadan year and bearing the same number, but beginning in july. It was in most parts of India Solar year, but different customs in different parts of india has made it divergent. There is Luni Solar and North west Fasali year.
Maharatta sur san or Shakur san
This is sometimes called Arabi san. It is extensively used in Mahratta domains. This is nine years behind Fasali of Deccan.
Magi San
This is used in Chittagong. It is similar to Benglai san. The only difference is it is 45 years behind.
Simha Samvat
This era is used in Kathiavad. THe era starts in 1036-37 Saka
Lakshmana Sena era
used in Tirhut and Mithila, but always with Saka or Vikrama era. Era starts in 1105-6 AD.
Ilaki Era
The Tarlkh-i-Illahi or Mighty Divine era was established by Akbar. It dates to his Accession 14 Feb 1556 (Friday 2nd Rabi-us-sani AH 963) . It is extensively used in Coins of Akbar and Jahangir, but Shah Jahan seems to discontinued.
Rajyabhisheka Saka
It is also called Mahratta Raja Saka Era. Saka here is era. This era was established by Shivaji on the day of accession (Jyeshta sukla Trayodasi (13th) of Saka 1596 expired or 1597, Ananda Samvatsara)
Pseudo-historical Era's
Old Saka Era
Some of the early Khartoshi and Brahmi insciptions cannot be dated to Saka or Vikrama era's. So they are classified to Old Saka Era distinguishing from Saka Era 78 AD. But there are many disagreements between scholars on which inscriptions should fall to Saka Era and Old Saka Era. Evenmore controversial is the exact origin of the said Era. And still more controversial is the exact date of the historical epochs mentioned with this Era. So this can be said to be one of the speculative Era.
Aguptaiyaka Era
The Gokak inscription dates itself to 845 th regnal year of unknown Aguptaiyaka kings. The inscriptionbelong to 6th century AD. So the era should start at 3rd or 2nd century BC. Speculation is it is connected with Chandragupta Maurya and local king Aguptaiyaka.
Other Era's
Some Inscriptions of Bengal are dated to Malla era starting at 694/695AD. Some Inscriptions of Bengal refer to Bengali Era (sana or Sala Era). Some orissa inscriptions refer to sana era as Amli era. There is also Siddha Hema Kumara Era(1142AD). Illahi Era (1556AD) instituted by Mughal Emperor Akbar. Some late inscriptions date themselves to Imgreji San or Isvi Era based on Christian era.
Phantom Era
Maurya Era
Some scholars studying the Hathigumpha Inscription of Kharvela have noted a Maurya Kala starting 324-300BC based on Chandragupta Accession. But sunsequent studies revealed that the inscription was stating Mukhya Kala (Sanskrit) not Maurya Kala. So that Era is hoax.
Astronomical
Kaliyuga Era
Kaliyuga era is supposed to begin with Kali aeon. Traditional associations with events Epics. Kaliyuga is creation of astronomers for chronological base for astronomical and calendrical calculations. The initial date corresponds to Friday, Feb 18 ,3102BC at the time of astronomical conjunction of Sun, Moon, Earth, and Planets at the point of naksatra mesa. Kali yuga is normally counted as expired. Earliest Kaliyuga era in inscription is Aihole inscription dated Kaliyuga 3735 and saka556 corresponding to 634 AD.
Saptarsi Era
Saptarsi Era also called Laukika,Sastra, Pahadi or Kacca is a imaginary cycle of 2700 years. The calculation is based on Saptarsi(Great Bear) which stands for 100 years in 27 nakshatras. Pahadi refers to the use in
hills of Punjab and Kashmir. Lukika and Kacca refers to omission of hundreds. Since it is mainly used in Astrology it is called sastra Samvat. The puranas give different dates and confuses this era.
hills of Punjab and Kashmir. Lukika and Kacca refers to omission of hundreds. Since it is mainly used in Astrology it is called sastra Samvat. The puranas give different dates and confuses this era.
Grahaparivriti Cycle
This is based on Kaliyuga Era , But instead of 100 year cycle it follows 90 year cycle. It is followed in Madurai region of tamil nadu. This year is fabricated by Astrologers of that area.
Brahaspati 12 Year Cycle
Brahaspati cycle is based on twelve year cycle of sidereal revolution of Jupitor. Some Gupta Feudatories , Kadambas, Rastrakuta used this in some inscriptions.
Brahaspati 60 Year cycle
This is originally calculated by jupitor transistion from one rashi to another. Due to discrepancy of four days every year, the cycle of requires suppression of one jupitor year every eightyfive years. This sixty year cycle is usually dated in name of the years then in numbers.
Ref
Indian Epigraphy by Richard Salomon.
Indian Epigraphy by D.C . Sircar.
Ancient Indian History and Civilization by Sailendra nath Sen.
The Indian Calendar, with Tables for the Conversion of Hindu and Muhammadan Into A.D. Dates, and Vice Versa Front Cover Robert Sewell, Robert Schram