Calendars used during the medieval period can be broadly categorized into systems of Hindu origin, Islamic origin and Roman origin, later adopted by Christianity. These systems were being used at the same time in different documents and geographies.

In many cases, multiple systems were being used by the same monarchy for different purposes. Many kings and emperors used to start new calendars based on the start of their rule over the kingdom.

All this creates huge complexity while deciding the dates of specific events and establishing their relation with other events.

Please also check out this Julian to Gregorian conversion link.

Let's take a brief look at the calendars and their usage.

Roman calendar was of Julian (Old Style) or Gregorian (New Style). The New Style was proposed by Pope Gregory in the 16th century. Both styles are based on solar year.

Hindu calendars were primarily the Shak Samvat, Vikram Samvat with their own variations and the Rajyabhishek Shak. They made use of both, solar year and moon movements.

Islamic calendars used were Hijri, Shuhur, Ilahi, Fasli, Julus and Turkish. Hijri system was based on lunar movement while few others were solar systems. Julus system was specific to a kings reign similar to Rajyabhishek Shak.

These systems had specific variations as per different geographies and that further increases the complexity for a student of history.

Shak Samvat was widely used in southern India. It started in year 78 of the Common Era (CE). One of the variations in this calendar was that in Maharashtra, completed year was counted while in Tamilnadu the current year was considered. This meant that Shak 1551 in Maharashtra would be Shak 1552 in Tamilnadu.

It also had differences in north (Pournimanta months) and south Indian (Amanta months) styles. Pournimanta refers to months ending on a full moon versus Amanta where month ends on a no-moon or new-moon day.

Vikram Samvat coincides with 57 BCE and is the official calendar of countries like Nepal. It is mostly used in the north Indian subcontinent.

Hijri Calendar (AH) is believed to have been started when Prophet Mohammad began his journey from Mecca to Madina. It was started by Khalif Omar in 639 CE, when Hijri year 17 was running. This system is purely Lunar based. Islamic day begins after sunset as against the Hindu day which begins at sunrise.

Shuhur calendar was introduced as Hijri calendar was not in sync with solar seasons. This was primarily used for calculating revenues from agricultural produce.

Fasli calendar was similar to Shuhur and was used for calculating revenues.

Ilahi calendar was started by Emperor Akbar on 11th March 1556 CE. This system was solar based. But it did not last in its original form beyond Akbar.

Roman calendar, now known as the Common Era (CE) was brought and widely used by Europeans who came to India. It used to record dates using the convention BC and AD to denote pre and post Christ dates respectively. Now these dates are denoted by Common Era (CE) and Before Common Era (BCE). But the Common Era dates were also plagued by differences between the Julian (Old Style) and Gregorian (New Style). For more information about the old and new styles please visit Old and New Style Calendars.

The difference between Old Style and New Style dates is essentially 10 days in the 17th century (upto 18th Feb 1700 O.S.), 11 days in the 18th century (18th Feb 1800 O.S.) and 12 days in the 19th century (16th Feb 1900 O.S.).