SQL - Add constraint for an existing column

-----alter an existing column to have default value
  update  tbl_name
  set appended = getdate()

  alter table  tbl_name
  alter column appended DATETIME NOT NULL ;

  ALTER TABLE tbl_name
  ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR appended;


-----get two digit month
 CONVERT(char(2), GetDate(), 101)