How does SqlDateTime do its precision reduction

Question

Consider the following program:

DateTime dateTime = new DateTime(634546165144647370);
SqlDateTime sqlDateTime = new SqlDateTime(dateTime);

Console.WriteLine         


        

Answer 1

SqlDateTime Structure

Represents the date and time data ranging in value from January 1, 1753 to December 31, 9999 to an accuracy of 3.33 milliseconds to be stored in or retrieved from a database. The SqlDateTime structure has a different underlying data structure from its corresponding .NET Framework type, DateTime, which can represent any time between 12:00:00 AM 1/1/0001 and 11:59:59 PM 12/31/9999, to the accuracy of 100 nanoseconds. SqlDateTime actually stores the relative difference to 00:00:00 AM 1/1/1900. Therefore, a conversion from "00:00:00 AM 1/1/1900" to an integer will return 0.

I would argue that sqlDateTime is within that 3.33 ms accuracy.

The DateTime value type represents dates and times with values ranging from 12:00:00 midnight, January 1, 0001 Anno Domini (Common Era) through 11:59:59 P.M., December 31, 9999 A.D.

As a side note, I saved this date time to a SQL Server database (in a DateTime column) and pulled it out again and it came out as 10:21:54.4670000. So I am really confused. (I thought SqlDateTime would match up with what SQL Server was going to do.)

Time values are measured in 100-nanosecond units called ticks, and a particular date is the number of ticks since 12:00 midnight, January 1, 0001 A.D. (C.E.) in the GregorianCalendar calendar (excluding ticks that would be added by leap seconds). For example, a ticks value of 31241376000000000L represents the date, Friday, January 01, 0100 12:00:00 midnight. A DateTime value is always expressed in the context of an explicit or default calendar.

and

Internally, all DateTime values are represented as the number of ticks (the number of 100-nanosecond intervals) that have elapsed since 12:00:00 midnight, January 1, 0001. The actual DateTime value is independent of the way in which that value appears when displayed in a user interface element or when written to a file. The appearance of a DateTime value is the result of a formatting operation. Formatting is the process of converting a value to its string representation.

Because the appearance of date and time values is dependent on such factors as culture, international standards, application requirements, and personal preference, the DateTime structure offers a great deal of flexibility in formatting date and time values through the overloads of its ToString method. The default DateTime.ToString() method returns the string representation of a date and time value using the current culture's short date and long time pattern. The following example uses the default DateTime.ToString() method to display the date and time using the short date and long time pattern for the en-US culture, the current culture on the computer on which the example was run.

It looks like DateTime is accurate within 100ns which is about the difference you have experienced.

As a side note, I saved this date time to a SQL Server database (in a DateTime column) and pulled it out again and it came out as 10:21:54.4670000. So I am really confused. (I thought SqlDateTime would match up with what SQL Server was going to do.)

This is also within the 3.33 ms accuracy...If you need something more accurate then 3.33ms then you will have to use SQL Server 2008