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