In ancient (and not so ancient) times, Jan 1 was not the beginning of the new year; a new year began on the vernal equinox, in March. So if you count from March, September is the seventh month, Oct. the 8th, etc. The change to Jan 1 being the first day of the new year came with the change from the Julian to the Gregorian calendars, so in England (and the American colonies) the switch occurred in (if I remember correctly) 1756. You will often see dates falling between Jan 1 and the March new year date shown with two years, i.e., Feb. 15, 1722/23, indicating that under the old system it was still 1722, but 1723 in the new system.