Update datetime values to same date but persist time - sql

Question

I'm in a situation where I would like to update the date of all the values in the table to have the same day but persist the time of each record.
So these three dates...
8/28/2012 14:00:00
8/28/2010 12:00:00
8/28/2008 10:00:00
Should be turned into...
8/28/2012 14:00:00
8/28/2012 12:00:00
8/28/2012 10:00:00
UPDATE MySpecialTable
SET DateField = {...?};
Edit: Regardless of the day the date is on (i.e. 8/28/2012, 1/1/2012, 4/1/2012, etc), I want to persist the time and mass update all the day/month/year to a specific value.

Answer 1

DECLARE #Target DATETIME;
SET #Target = '20120828';
-- if the date is supposed to be today rather than hard-coded or parameterized:
-- SET #Target = DATEADD(DAY, DATEDIFF(DAY, '19000101', CURRENT_TIMESTAMP), '19000101');
UPDATE dbo.MySpecialTable
SET DateField = DATEADD(DAY, DATEDIFF(DAY, DateField, #Target), DateField);
This will work if the target date is before or after the value in the column.
Also, if you only care about time, you should consider using the TIME data type (SQL 2008+). This will make it easy to apply any date you want to the data at query time, without having to actually perform any updates.

Answer 2

Here is one way, assuming you are using SQL Server 2008 or greater:
update MySpecialTable
set DateField = cast(CURRENT_TIMESTAMP as date) + (DateField - cast(DateField as date))

Answer 3

Try this (updated to have the same date):
DECLARE #myDate DATETIME
SET #myDate = '2012-08-28T00:00:00.000'
UPDATE [MySpecialTable]
SET [DateField] = DATEADD(Day, 0-DATEDIFF(Day, 0, [DateField]), [DateField]) + #myDate
DATEADD(Day, 0-DATEDIFF(Day, 0, [DateField]), [DateField]) gets the
timestamp and makes the date to the min value (1/1/1900)
Add the above to the desired date in the variable