>I suspect some dates have time in them and also Michel is using floating math by using 86400.00, for example. Try that for your example.

From the same sample code I had, if I change it to

SELECT [Holiday].[Name], SUM(DATEDIFF(s, [Holiday].[Start], [Holiday].[End]))/86400.00 from @Holiday Holiday group by [Holiday].[Name]

I've got:

-- Christmas	365.00000000
-- New Year	365.00000000

>
>

Changing one record to have the time portion too:

declare @Holiday table ([Name] varchar(32), [Start] datetime, [End] datetime) 

insert into @Holiday ([Name], [Start], [End]) values ('Christmas', '2014-12-24 06:00:00', '2015-12-24 03:00:00')
insert into @Holiday ([Name], [Start], [End]) values ('New Year', '2014-01-01', '2015-01-01')

SELECT [Holiday].[Name], SUM(DATEDIFF(d,[Holiday].[Start], [Holiday].[End])) from @Holiday Holiday group by [Holiday].[Name]

SELECT [Holiday].[Name], SUM(DATEDIFF(s, [Holiday].[Start], [Holiday].[End]))/86400.00 from @Holiday Holiday group by [Holiday].[Name]

I've got

-- Christmas	365
-- New Year	365

-- Christmas	364.87500000
-- New Year	365.00000000

Which looks ok to me, but then again, I might be missing Michel point altogether