To add or subtract days from a date, you can simply use:

-- © 2011 – Vishal (http://SqlAndMe.com)
 
SELECT      GETDATE() AS 'Today',
            GETDATE() + 10 AS '10 Days Later',
            GETDATE() - 10 AS '10 Days Earlier'

Result Set:

Today                   10 Days Later           10 Days Earlier
———————–                ———————–                ———————–
2011-05-20 21:04:24.527 2011-05-30 21:04:24.527 2011-05-10 21:04:24.527
 
(1 row(s) affected)

However, if you need to add months/years to date, you need to use DATEADD() function.

Syntax: DATEADD( datepart, number, date)
Where, datepart = year, quarter, month, day… (Check BOL for full list)
             number = is a integer values that is to be added to the “datepart” of date
             date = date

It can be used as:

-- © 2011 – Vishal (http://SqlAndMe.com)
 
SELECT      GETDATE(), 'Today'
UNION ALL
SELECT      DATEADD(DAY,  10, GETDATE()), '10 Days Later'
UNION ALL
SELECT      DATEADD(DAY, -10, GETDATE()), '10 Days Earlier'
UNION ALL
SELECT      DATEADD(MONTH,  1, GETDATE()), 'Next Month'
UNION ALL
SELECT      DATEADD(MONTH, -1, GETDATE()), 'Previous Month'
UNION ALL
SELECT      DATEADD(YEAR,  1, GETDATE()), 'Next Year'
UNION ALL
SELECT      DATEADD(YEAR, -1, GETDATE()), 'Previous Year'

Result Set:

———————–                —————
2011-05-20 21:11:42.390 Today
2011-05-30 21:11:42.390 10 Days Later
2011-05-10 21:11:42.390 10 Days Earlier
2011-06-20 21:11:42.390 Next Month
2011-04-20 21:11:42.390 Previous Month
2012-05-20 21:11:42.390 Next Year
2010-05-20 21:11:42.390 Previous Year
 
(7 row(s) affected)

Hope This Helps! Cheers!