On 2nd September 1752, in England and her dominions, the Gregorian calendar that we use now was officially adopted, in place of the Julian calendar. The day following 2nd September became the 14th but, for the genealogist, there's more of a problem than just the 12 missing days. The date of the new year was officially moved from 25th March to 1st January. Before the change, the last day of 1750 was 24th March and the first day of 1751 was 25th March (nine months before Christmas Day).
When researching family history dates will often be seen from 1st January to 24th March (for years before 1752) written with a "dual" year e.g. 5th March 1691\2 indicates 1691 on the Julian calendar and 1692 on the Gregorian calendar. Dates after 24th March (e.g. 6th May 1601) do not have a "dual" date as they can only fall in one year.
If a date before September 1752 is encountered during research without a dual date and it falls between 1st January and 24th March, check the original source.
Follow this link for online Julian and Gregorian calendars.