SQL-Server: Exploration of datetime2(7) precision

Introduction

In this article with code samples, learn about using datetime2 precision.

What is datetime2?

Defines a date that is combined with a time of day that is based on 24-hour clock. datetime2 can be considered as an extension of the existing datetime type that has a larger date range, a larger default fractional precision, and optional user-specified precision.

Precision scale

We tend to define a datetime2(7) where in this case 7 is the precision and can range from 0 to 7. This means in a table if the default 7 is not need, use the precision which works best for given business requirements or leave as 7.

Typically developers don’t need to be concerned with milliseconds part of a date but when needed using datetime2 provides milliseconds. Write a query with a column of type datetime2 and milliseconds are display. Now write code using C# to return data from the same query to a DataTable, DataSet, DataReader or Entity Framework and note it appears milliseconds has been truncated. Below learn how to get full milliseconds in C# no matter how the data is retrieved and how to format milliseconds in a user interface.

Diving in working with datetime2 precision

When working with datetime2(7) in a SQL-Server table we see time as hours, minutes, seconds and milliseconds.

Example table definition

Write a simple SELECT statement.

SELECT Id,
       [User],
       Created
FROM dbo.AuditLog;

Returns

Now we know that milliseconds are properly stored, time to read the records into a C# application using either a DataReader, DataTable or Entity Framework Core and for the first record the expected value for Created column should be 2022-11-26 17:44:28.4006356 but what the DataReader, DataTable or Entity Framework Core returns is 2022-11-26 17:44:28.400. This can be frustrating to a developer that does not understand what is happening.

If we look at TimeOfDay.TotalSeconds the fraction is out milliseconds. Knowing this let's create a languge extension to get milliseconds, the fraction part of TotalSeconds.

public static class Extensions
{
    public static decimal GetMilliseconds(this double sender) 
       => Convert.ToDecimal(sender) % 1.0m;
}

Well that is partly there, let's get the method to return an int rather than a decimal

public static class Extensions
{
    public static int GetMilliseconds(this double sender) 
        => Convert.ToInt32(Convert.ToString(Convert.ToDecimal(sender) % 1.0m, 
               CultureInfo.InvariantCulture).Replace("0.", ""));
}

Usage

int milliseconds = created.TimeOfDay.TotalSeconds.GetMilliseconds();

✔️ Wait, we can do even better. Note .ToString is used as we do not want to add Milliseconds and Milliseconds, that is incorrect, instead perform string concatenation.

public static class Extensions
{
    public static int GetMilliseconds(this DateTime sender) 
        => Convert.ToInt32(sender.TimeOfDay.Milliseconds.ToString() + 
               sender.TimeOfDay.Milliseconds.ToString());
}

Perhaps one more update, if the requirements is to get precision to 7 the last method does not but the following does.

public static class Extensions
{
    public static int GetMilliseconds7(this DateTime sender) 
        => Convert.ToInt32((sender.TimeOfDay.Milliseconds.ToString() + sender.TimeOfDay.Microseconds.ToString())
            .PadRight(7, '0'));
}

In this case we use

created.GetMilliseconds();

created.GetMilliseconds7();

Formatting

If all that is needed is to format milliseconds and the precisions is unknown. First query the database using the following statement in SSMS.

SELECT 	TABLE_NAME,COLUMN_NAME,DATETIME_PRECISION FROM INFORMATION_SCHEMA.COLUMNS WHERE  DATA_TYPE = 'datetime2';

For C# provided in the project SqlServerLibrary and used in the project SqlServerDateTime2PrecisionApp with a DataReader, DataTable and EF Core 7.

public static (List<DateTimeInformation> list, bool hasColumns) GetDateTimeInformation(string connectionString, string tableName)
{
    List<DateTimeInformation> dateTimeInfoList = new();
    var sql =
        "SELECT TABLE_NAME,COLUMN_NAME,DATETIME_PRECISION " + 
        "FROM INFORMATION_SCHEMA.COLUMNS WHERE DATA_TYPE = 'datetime2' AND TABLE_NAME = @TableName;";

    using var cn = new SqlConnection(connectionString);
    using var cmd = new SqlCommand(sql, cn);
    cmd.Parameters.Add("@TableName", SqlDbType.NChar).Value = tableName;

    cn.Open();

    var reader = cmd.ExecuteReader();
    if (reader.HasRows)
    {
        while (reader.Read())
        {
            dateTimeInfoList.Add(new DateTimeInformation()
            {
                TableName = reader.GetString(0), 
                ColumnName = reader.GetString(1), 
                Precision = reader.GetInt16(2)
            });
        }

        return (dateTimeInfoList, true);
    }
    else
    {
        return (null, false)!;
    }
}

Documentation

Microsoft docs for datetime2

Working with date time
Working with DateOnly primer
EF Core: Working with TimeOnly
SQL-Server: Computed columns which includes working with dates.
Writing SQL for your application

Summary

With what has been presented code provided shows how to get milliseconds from a datetime(n) along with how to format datetime(n) where the n could be 7 or perhaps 4.