Your ancestor was baptised on 2 February 1730/31 — what does the slash mean?

If you have ever met a date like 12 March 1745/6 in a parish register or a will, you have met the calendar change that quietly trips up British and colonial American research.

Until 1752, the legal year in England and its colonies began on 25 March, not 1 January. So a baptism on 2 February 1730 in the register was, to a modern reader, 2 February 1731. Careful clerks and later transcribers wrote "1730/31" to flag it.

The fix came with the Calendar (New Style) Act 1750 (24 Geo. II c. 23). Its text on legislation.gov.uk says the days were counted as usual up to 2 September 1752, and the next day was to be counted as 14 September. It also moved the start of the year to 1 January from 1752.

Two practical traps for family historians:

• Jan–March dates before 1752 are the ones to double-check. A child who seems to be born before the parents' marriage may simply be a year off.
• Whether a transcription already "corrected" the year is not always stated. It is worth viewing the original image when you can.

Question for the genealogists here: have you had a brick wall that turned out to be a Jan–Mar date, or a record where the dual-date notation was written differently (e.g. just "1730" when the clerk plainly meant 1731)? And how do you record such dates in your own trees?