You can use a constant expression as a default value, as long as the expression does not reference database objects.
You can use functions such as GETDATE and DATEADD in a constant expression default value. If the default constant expression is not a function or simple value, you must enclose the expression in parentheses.
For example, constant expressions allow column defaults to contain entries such as the date fifteen days from today:
... DEFAULT ( DATEADD( DAY, 15, GETDATE() ) )