Before the calendar changes of 1752, the new year began not on 1 January but on Lady Day, which was 25 March. Thus January and February 1603 for example and the first 24 days of March 1603 came after December 1603: December 1603 January 1603 February 1603 1-24 March 1603 then the next day is 25 March 1604 To avoid confusion over dates in January/February/1-24 March before 1752 it is customary to use 'double dating', so an event that occurred in the January after December 1603 would be written as January 1603/4. Unfortunately many entries in FamilySearch and elsewhere do not use this convention. When you are just presented with a single entry out of context, it can be hard to be sure of the exact date. To be sure which year is really intended it is crucial to see where the entry fits in the sequence of entries in the parish register.