"Table of human conception in two adjacent, concentric circles divided into 12 segments; outermost circle, around edge, with date of conception, read first; inner circle gives date of birth and number of days to add to day of conception ... The calendar helps to calculate the day of birth from the date of conception, assuming an average human gestation period of 280 days. The first row gives the month of conception, the second the month of birth with the number of days to be added to the day of conception. Thus the birth of a child conceived on 1 January will be expected around 8 October."--British Museum.