TSQL - Greatest of two dates using arithmetic

declare @val1 as date;

declare @val2 as date;

set @val1='2011-12-31';

set @val2='2012-06-27';

select

       DATEADD

       (

              d

              ,

              cast(

                     0.5

                     * (

                           (

                                  datediff(DAY,'1990-01-01',@val1)

                                  + datediff(DAY,'1990-01-01',@val2)

                           )

                           + ABS(

                                  datediff(DAY,'1990-01-01',@val1)

                                  - datediff(DAY,'1990-01-01',@val2)

                           )

                     )

                     as int

              )

              , cast('1990-01-01' as date)

       )

;