While the ways of expressing dates and the complications caused by the use of the Julian calendar provide multiple opportunities for historical confusion, people in Early Modern England had one other way of puzzling today's reader: regnal years.
What this means is that official papers, and other documents where dates were written down, regularly expressed the year as the regnal year of a specific monarch. So a document might give a date as 'the 10 day of December in the 14 year of the Reign of our Sovereign Lord Charles the II. by the Grace of God, King of England, Scotland, France and Ireland, Defender of the Faith, &c.'. In this case the author provides additional help by identifying the year as 1662. But that is by no means universal. For example, the document from which this line was taken also referred to 'the 43 year of the Reign of the late Queen Elizabeth' without providing a helpful date.
So historians often need to know when each monarch ascended the throne. However, the additional complication is that the regnal year runs from the day of accession and not from either Lady Day (25 March, the start of the year until 1752) or 1 January.
The other point that is relevant to those interested in the English Civil Wars period is that official documents ignore the existence of the Commonwealth period. As can be seen above, the reign of Charles II was defined as starting on the death of his father, Charles I and therefore, 1662 became 'the 14 year of the Reign of our Sovereign Lord Charles the II' even though he was not restored as Monarch until 1660.