Computed parameters in Stored Procedure

 You are aware of Computed Columns in SQL Server. In a similar way you can also use Computed Parameters. Consider the following procedure.

1.Create procedure test

2.(

3.@datetime datetime,

4.@date date = @datetime

5.)

6.as

7.select @datetime as date_with_time, @date as date_only

When you execute the procedure you get this result

1.EXEC '2001-10-18 12:45:22.000'

1.date_with_time          date_only

2.----------------------- ----------

3.2001-10-18 12:45:22.000 2001-10-18

As you see the parameter accepts the value supplied for @datetime and convert it to date datatype. 
When you supply value for @date, it will ignore the value of @datetime, the default value for it

view source

print?

1.EXEC '2001-10-18 12:45:22.000','2012-01-22 19:15:12.000'

1.date_with_time          date_only

2.----------------------- ----------

3.2001-10-18 12:45:22.000 2012-01-22

However it is not possible to use an expression for the computed paramters 
ie @date date=dateadd(day,10,@datetime) will not work. You should be aware of this when you use paramers
this way.