Using function as a parameter when executing a stored procedure?

You can't use a function directly as a stored procedure parameter.

You can do the following:

DECLARE @now DateTime
SET @now = GETDATE()

DECLARE @return_value int
EXEC @return_value = my_stored_procedure
        @MyId = 1,
        @MyDateField = @now
SELECT  'Return Value' = @return_value
GO

per MSDN

Execute a stored procedure or function
[ { EXEC | EXECUTE } ]
    { 
      [ @return_status = ]
      { module_name [ ;number ] | @module_name_var } 
        [ [ @parameter = ] { value 
                           | @variable [ OUTPUT ] 
                           | [ DEFAULT ] 
                           }
        ]
      [ ,...n ]
      [ WITH RECOMPILE ]
    }
[;]

    Execute a character string
    { EXEC | EXECUTE } 
        ( { @string_variable | [ N ]'tsql_string' } [ + ...n ] )
        [ AS { LOGIN | USER } = ' name ' ]
    [;]

    Execute a pass-through command against a linked server
    { EXEC | EXECUTE }
        ( { @string_variable | [ N ] 'command_string [ ? ]' } [ + ...n ]
            [ { , { value | @variable [ OUTPUT ] } } [ ...n ] ]
        ) 
        [ AS { LOGIN | USER } = ' name ' ]
        [ AT linked_server_name ]
    [;]

Notice for @parameter you can either specify a value or a variable or specify Default. So you got to set the value of a variable as GetDate() (as others have specified) and use that variable.

HTH