SQL Server 2008 and milliseconds

Question

In SQL Server 2008, why do the following queries return the same value?

-- These all return 2011-01-01 23:59:59.997
SELECT CAST('2011-01-01 23:59:59.997' as datetime)
SELECT CAST('2011-01-01 23:59:59.998' as datetime)

And why does the following query round to the next day?

-- Returns 2011-01-02 00:00:00.000
SELECT CAST('2011-01-01 23:59:59.999' as datetime)

Answer 1

The accuracy of DateTime within SQL Server has always been to 1/300s of a second (3.33ms), so any value that does not divide precisely gets rounded.

• 997 stays as it is
• 998 will round to 997
• 999 will round up to 000

To get additional accuracy, there is the DateTime2 data type, available in SQL Server 2008 onwards, that can be accurate to 7 decimal places.

Answer 2

The MSDN docs for datetime at http://msdn.microsoft.com/en-us/library/ms187819.aspx say

Time range == 00:00:00 through 23:59:59.997 Accuracy == Rounded to increments of .000, .003, or .007 seconds

In the linked document there is also a section "Rounding of datetime Fractional Second Precision".

datetime2 gives you more accuracy.