Question

I’ve got a bunch of datetime values where I need to replace the date part with a specific date passed as a parameter (an nvarchar(8) formatted as YYYYMMDD). The time part should stay same as before.

For instance, I’ve got this row of datetime values:

Id | DocDate                 | CreationDate            | PrintDate 
1  | 2012-10-01 00:44:20.150 | 2012-10-07 00:44:20.150 | 2012-10-07 00:50:20.150

If I’m passing '20121005', the values should change like this:

Id | DocDate                 | CreationDate            | PrintDate
1  | 2012-10-05 00:44:20.150 | 2012-10-05 00:44:20.150 | 2012-10-05 00:50:20.150

How can I do that?

Answer 1

DECLARE @T TABLE (id int, docdate datetime,
                  creationdate datetime,
                  printdate datetime);
INSERT @t SELECT
 1, '2012-10-01 00:44:20.150',
    '2012-10-07 00:44:20.150',
    '2012-10-07 00:50:20.150';

DECLARE @newdate datetime = '20121005';

UPDATE @T SET
  docdate = dateadd(d,datediff(d,docdate,@newdate),docdate),
  creationdate = dateadd(d,datediff(d,creationdate,@newdate),creationdate),
  printdate = dateadd(d,datediff(d,printdate,@newdate),printdate);

SELECT * FROM @t;

The pattern of changing just the date portion of a datetime is to first find the difference (in days) that you need to apply, then apply it.

 e.g. datediff(d, '20121001', '20121008') => +7 days
      dateadd(d, <+7>, '20121001 xx:xx') => add the 7 days, keeping time