with jul_dat as
(
    select dd1 / 1000 yyyy,
           dd1 % 1000 days_since_1,
           cast( cast(dd1 / 1000 as varchar) + '-01-01' as datetime) jan_1_of_year
    from
    (
        select 9999999 - @dat dd1
    ) x
)
select jan_1_of_year + days_since_1 - 1 from jul_dat