One problem that researchers face when looking at historic documents is the accurate dating of each manuscript. Modern dating systems simply do not apply when it comes to the Medieval World. Indeed, until the mid 18th century, the year itself was documented in a different way. 

This document from the University of Nottingham acts as an introduction to the issue of dating historic documents. It is not specific to the Wars of the Roses, though the issues raised do apply to this era.

Dating Documents

Things that are commonly found in documents, which are explained very well in the skills document, include:

• Dating by Regnal Year. Many documents are listed as being in the xth year of the reign of a monarch. This can present problems when dating a source. The document outlines how best to tackle this.
• Differences between the Civil and Ecclesiastical years. They haven’t always run alongside each other. And the concept of beginning the year on 1st January is quite recent in terms of having uniformity to it. So it is quite possible to find a document that could be accurately dated to two years, depending on which calendar is being used. *This is true of dates from 1st January to 25th March of each year up to 1752 (in England, Ireland, Scotland and Wales).
• There has been a change from the Julian to Gregorian calendar. The dating on one does not match with that on the other. So, there needs to be consistency in the way in which the dates from the pre-Gregorian period are translated into the current dating system.
• Dating through reference to Holy Days is very common in Medieval documents. Unfortunately, this is also quite troublesome. Many religious feasts are moveable, they do not occur on the same day and month each year. The stand out example is Easter, but the annual celebration of this feast determines the day upon which many other feasts will fall within that ecclesiastical year.
• Sources that originate from overseas or nationals from other states may use different dating systems. England and Wales had their own civil year. So too did other European states. They do not necessarily match up so not only have you got conflicting dating structures within England and Wales but also potentially additional ones for sources that originate from traders, ambassadors or religious orders who visited.
• Dated by Terms. These are still in use in some aspects of life. Most commonly associated with the Legal system and older Universities, dates may refer to Hilary, Micklemass or Trinity Terms.
• It is also worth remembering that dates can still be written in a variety of ways. Different faiths have different calendars. Use of Roman Numerals remains, alongside the use of Arabic Numerals. AD can be used, as can CE. Different translations of older texts will therefore be adding a range of modern styles, not just the one with which you are most familiar.