A separate issue from Old Style vs New Style calendars — though often conflated because England adopted both changes around the same time — is the date on which the new year began.
Today the new year starts on 1 January (Modern Style). Before 1752 in England, it started on 25 March (Lady Day or Annunciation Style). Scotland adopted Modern Style year numbering earlier, in 1600. Other countries changed at various times — France, for example, standardised on 1 January from 1564, having previously used different dates in different regions.
To make the year numbering unambiguous, it is customary to use dual years for dates falling between 1 January and 24 March in periods when the new year started later:
2 Feb 1644/5
This indicates that the event was recorded as 1644 in the original document but is 1645 in Modern Style numbering. If only a single year is given, it is assumed the conversion to Modern Style has already been applied.
Note — Dual dates are supported in GEDCOM 5.5.1 but not in GEDCOM 7.0. When exporting to version 7.0, dual dates are exported as a phrase with the modern (higher) year used for sorting.