Undesired rounding of DateTime in SQL Server - sql-server

Question

I ran into something that seems odd. SQL Server appears to be rounding some DateTime values inappropriately when I save them to datetime columns. I suspect I'm missing something, but I can't spot it. I'm running this test against SQL Server 2008 using .NET 4.0. The following should illustrate the issue:
I have created a table in in SQL Server called Timestamps. It has two columns:
id - bigint, Identity, PK
timestamp - datetime
I also created a simple test that does the following:
Gets the current time, truncating the value to millisecond precision
Saved the truncated time to Timestamps
Retrieved the datetime` value from the DB and compared it to the original (truncated) DateTime object.
public static void RoundTest()
{
DateTime preTruncation = DateTime.UtcNow;
DateTime truncated = preTruncation.TruncateToMilliseconds();
using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["test"].ConnectionString))
{
conn.Open();
SqlCommand cmd = new SqlCommand(#"INSERT INTO Timestamps(timestamp)
VALUES(#savedTime);
SELECT SCOPE_IDENTITY() AS id");
cmd.Parameters.Add(new SqlParameter("savedTime", truncated));
cmd.Connection = conn;
var id = cmd.ExecuteScalar();
SqlCommand get = new SqlCommand(#"SELECT timestamp FROM Timestamps
WHERE id = #id");
get.Parameters.Add(new SqlParameter("id", id));
get.Connection = conn;
DateTime retrieved = (DateTime)get.ExecuteScalar();
if (retrieved != truncated)
{
Console.WriteLine("original: " + preTruncation.TimeOfDay);
Console.WriteLine("truncated: " + truncated.TimeOfDay);
Console.WriteLine("retrieved: " + retrieved.TimeOfDay);
Console.WriteLine();
}
}
}
Although I expect the truncated value to be equivalent to the value returned back from the DB, that is not always the case. Here's some sample output:
original: 19:59:13.4049965
truncated: 19:59:13.4040000
retrieved: 19:59:13.4030000
original: 19:59:14.4989965
truncated: 19:59:14.4980000
retrieved: 19:59:14.4970000
original: 19:59:15.4749965
truncated: 19:59:15.4740000
retrieved: 19:59:15.4730000
original: 19:59:30.1549965
truncated: 19:59:30.1540000
retrieved: 19:59:30.1530000
TruncateToMilliseconds() looks like this:
public static DateTime TruncateToMilliseconds(this DateTime t)
{
return new DateTime(t.Year, t.Month, t.Day, t.Hour, t.Minute, t.Second, t.Millisecond);
}
What gives? Is this really inappropriate rounding, or am I making a mistaken assumption here?

Answer 1

Datetime is only accurate to 3ms. Therefore it'll round to the nearest multiple of 3ms. To overcome this, look at the datetime2. Note that this is for SQL2008+ only
EDIT: it's not quite only to 3ms. It's rounded to increments of of .000, .003, or .007 seconds