In 1582 Pope Gregory XIII ordered that 10 days be removed from the month of October for reasons too complicated to bother with and thus our modern (Gregorian Calendar) came into being.
Britain was Protestant by then and decided not to follow the Pope’s changes and kept the ‘old calendar’ for another 170 years, finally changing to the Gregorian calendar – the one we use today. That meant that many legal documents prior to the change had to have both an ‘OS’ and a ‘NS’ dates (old style and new style). Ultimately the calendar adjustment meant that in fact 12 days were lost in the creation of the new calendar.
Therefore if we want to know what date it would be by the OS calendar we add on 12 days to the NS calendar. Therefore October 31st by the NS will be November 12th OS.
Now when we consider the Charles Walton murder date of February 14th 1945 NS that would be Candlemas by the OS calendar.
If we wanted St Valentine’s day by the O.S it would be February 26th by the NS.