Select dateadd(day,21,existing_date_column)

From table.

Above query add 21 days to existing_date_column and display its result.

You can also add

Year,

quarter,

Month,

dayofyear,

Day,

Week,

Hour,

minute,

second,

millisecond

ex.

Select dateadd(year,1,existing_date_column)

From table.

It will add 1 year in existing_date