Update only time from my Datetime field in sql

Question

I have a date 2013-12-14 05:00:00.000 in my table.

Now i want to update only time of that date. E.G to 2013-12-14 04:00:00.000

Is there any query to update o

Answer 1

DECLARE @d datetime;
SELECT @d = DATEADD(dd, DATEDIFF(dd, 0, GETDATE()), 0);

DECLARE @t time(7);
SET @t = CAST('10:10:10' AS time);

SELECT CONVERT(datetime, CONVERT(varchar(10), CONVERT(date, @d, 101)) + ' ' + CONVERT(varchar(8), @t));

Answer 2

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(CAST(MyDate AS DATE) AS DATETIME)) 

Or this

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(FLOOR(CAST(MyDate AS FLOAT)) AS DATETIME))