@DATE(year;month;day) calculates the date number for the specified year, month, and day.
A date number is a number from 1 through 2958525 assigned in sequence to each date from January 1, 1900, through December 31, 9999. For example, the date number for July 21, 1991, is 33440.
year is an integer from 0 (the year 1900) through 8099 (the year 9999).
month is an integer from 1 through 12.
day is an integer from 1 through 31. The value you use for day must be a valid day for the month. For example, you cannot use 31 as the day if you use 4 (April) as the month.
Even though February 29,1900, did not exist (it was not a leap year), it is assigned a date number. This does not invalidate any of your date calculations unless you use dates from January 1, 1900, through March 1, 1900. If you are using dates within that period, subtract 1 from the date number within the period.
If you want the results of an @DATE calculation to appear as an actual date, format the cell that contains the @DATE function with one of the date formats.
@DATE(92;2;21) returns 33655, or 21-Feb-92, in a cell formatted as day-month-year.
@DATE(91;2;29) returns ERR, because 1991 was not a leap year.