Unexpected rounding-off from DATETIME2 to DATETIME

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SQL-Server-2008R2

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First of all, it may not have a practical use but just want to know how/why this is happening.

When I am trying to assign a DATETIME2 value to a DATETIME variable, there happens an unexpected roundoff in the value.

Sample SQL :

SQL

DECLARE @DT DATETIME
DECLARE @DT2 DATETIME2

SET @DT2='2015-07-23 18:35:34.8815038'
SELECT @DT=@DT2

SELECT @DT2 AS [DATETIME2],@DT AS [DATETIME]

OUTPUT :

DATETIME2	                         DATETIME               
2015-07-23 18:35:34.8815038    |     2015-07-23 18:35:34.883

Now, I am not able to find the cause why 8815038 nano seconds is rounded off to 883 milliseconds against expected 881 or 882 milliseconds.

If any body knows the exact explaination to this, please share here. :)

Posted 23-Jul-15 3:27am

Suvendu Shekhar Giri

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Wendelius · Accepted Answer · 2015-07-23T03:40:00

Datetime2 is more accurate than datetime which has only accuracy for milliseconds.

From datetime2 (Transact-SQL)[^]:

When the fractional precision of the datetime2(n) value is greater than three digits, the value will be truncated.
The following example shows the results of converting a datetime2(4) value to a datetime value.

SQL
DECLARE @datetime2 datetime2(4) = '1968-10-23 12:45:37.1237';
DECLARE @datetime datetime = @datetime2;

SELECT @datetime AS '@datetime', @datetime2 AS '@datetime2';

--Result
--@datetime               @datetime2
------------------------- ------------------------
--1968-10-23 12:45:37.123 1968-10-23 12:45:37.1237
--
--(1 row(s) affected)

What comes to the value not being 881 milliseconds this happens because of the accuracy of datetime. Datetime is not accurate on millisecond level but Accuracy is rounded to increments of .000, .003, or .007 seconds;

You can visualize this with a small loop:

SQL

DECLARE @DT DATETIME
DECLARE @DT2 DATETIME2
DECLARE @counter INT = 0
 
SET @DT2='2015-07-23 18:35:34.8815038'

WHILE @counter < 10 BEGIN
   SELECT @DT=@DT2
   SELECT @DT2 AS [DATETIME2],@DT AS [DATETIME];
   SET @DT2 = DATEADD(ms, 1, @DT2)
   SET @counter = @counter + 1
END

For more information, see datetime (Transact-SQL)[^]

Suvendu Shekhar Giri · Accepted Answer · 2015-07-23T03:57:00

Here I got the solution. I have posted the same question in Stackoverflow[^]

Quote:
The reason is that in datetime type, you only have .000, .003 and .007 fraction of a second.

Try this:

SQL

select cast('2015-01-01 00:00:00.882' as datetime)
select cast('2015-01-01 00:00:00.884' as datetime)

And you'll see that both are rounded to .883
Accuracy of datetime is rounded to increments of .000, .003, or .007 seconds.

Reference:
https://msdn.microsoft.com/en-us/library/ms187819.aspx[^]

Unexpected rounding-off from DATETIME2 to DATETIME

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