date: the particular day, month, or year (usually according to the Gregorian calendar) that an event occurred
calendar year: the year (reckoned from January 1 to December 31) according to Gregorian calendar
acquired immunity: immunity to a particular disease that is not innate but has been acquired during life; immunity can be acquired by the development of antibodies after an attack of an infectious disease or by a pregnant mother passing antibodies through the placenta to a fetus or by vaccination