ADDDAY adds the specified number of days (as set with the int parameter) to a date or datetime value. This function will always return a valid date or datetime if the beginning date or datetime is valid. If necessary the date will be "rolled" to the next higher value. For example adding 34 days will advance the month as well as adding the correct number of days (depending on the number of days in the months in question).
In the following example the value of vNewDate would be February 3rd, 2000 (2/3/2000).
SET VAR vNewDate DATE = (ADDDAY('1/27/2000',7))
Using a negative INT parameter will result in subtracting that many days.