The result of [Datetime + 420./1440] (7 hours) is wrong (precision in ms is lost)

Answer 1

Actually, the even correcter way of adding 7 hours to a datetime is to use DATEADD:

select getdate() as dt into #t1
select dt, dateadd(hh, 7, dt) as dt_new from #t1

gives

dt                      dt_new
----------------------- -----------------------
2010-10-25 13:16:35.067 2010-10-25 20:16:35.067

Answer 2

You could also use the inbuilt functions, such as

select dt, DATEADD(hour, 7, dt) as dt_new from #t1

My guess as to the imprecision in using + is that its converting from datetime to integer, then back again.

Answer 3

You could achieve better result with dateadd() function (like previous posters noted). If you want use arithmetics, use better datatype, like float - cast(7 as float) instead of 7. is working well.