>The problem here is indirect casting to float and therefore wrong calculations. Try casting to DECIMAL to obtain correct result, e.g.
>
>

>;with cteDates as (select dateadd(day, n.number, '20000101') as [startDate],
>dateadd(day, n.number, '20000102') as [endDate]
>from dbo.numbers N where N.number  between 0 and 364)
>
>select SUM(datediff(day, startDate, endDate)) as [Days Count],
>SUM(cast(datediff(second, startDate, endDate)  as decimal(12,2))/86400)  as [Days /Seconds Count],
>SUM(datediff(second, startDate, endDate))/86400.00  as [Wrong Days /Seconds Count]
>from cteDates 
>

Thanks