It’s likely you will have come across dates where the transcribed year appears a little ambiguous, written in the format 1748/49. The reason for this isn’t (usually) that the transcriber struggled to read the date, but rather that the year is being displayed using both the Julian and Gregorian calendars.
In 1752, England, Wales, Scotland and Ireland adopted the Gregorian Calendar (the calendar we use today), prior to that the Julian Calendar was used. When reading old records, it’s important to remember that the Julian Calendar’s year began on 25th March, not the 1st January.
The relevance of this to genealogists
It’s natural for us today to glance at dates and see December as the end of the year, but prior to 1752 the end of the year was March.
You will often see dates recorded in transcripts in the format of, for example, 1748/49. This doesn’t (usually) mean that the transcriber couldn’t read the date, it means that the event took place between 1st January and 24th March 1748 (what we would call 1749 if we converted it to the Gregorian Calendar).
It can be confusing, here’s an example
If a child was born on 25th February 1748 and baptised on 27th March 1749 it can appear that the parents waited over a year for the baptism, when in fact the baptism took place when the baby was 4 weeks old (remember, New Year’s Day was 25th March). To ensure that this is quickly understood, it’s common to record the date of birth as 25th February 1748/49 (1748 being the Julian year, the year the even actually happened, and 1749 being the converted Gregorian date) and the baptism as 27th March 1749 (because it occurred after 25th March there’s no need to convert the year).