@DAY(date-number) extracts the day of the month, a value from 1 through 31, from date-number.
date-number is a value from 1 (January 1; 1900) through 2958525 (December 31; 9999).
You can use one of the other date @functions to supply the value for date-number.
@DAY can supply the day argument for other date @functions that build on previously calculated dates.
@DAY(@TODAY) = the current day of the month.
@DAY(D9) = 12, if cell D9 contains the date number 33250 (the date 12-Jan-91).