DATEADD adds an interval to a date you specify. For example, if the publication dates of all books in the titles table slipped three days, you could get the new publication dates with this statement:
SELECT DATEADD(day, 3, pubdate) FROM titles
If the date parameter is a smalldatetime datatype, the result is also a smalldatetime. You can use DATEADD to add seconds or milliseconds to a smalldatetime value, but doing this is meaningful only if the result date returned by DATEADD changes by at least 1 minute.