Why Does SQL Server Lose Milliseconds When Storing DateTime Data?

SQL Server's DateTime Data Type: The Missing Millisecond

Data accuracy is critical, especially when dealing with timestamps. However, SQL Server's datetime type presents a common problem: the loss of milliseconds. This article explores why this occurs and offers solutions.

Consider this scenario: data is inserted using:

INSERT INTO TESTTABLE (IntField, BitField, StringField, DateField) 
VALUES ('1', 1, 'hello', {ts '2009-04-03 15:41:27.378'});

Retrieving the data with:

select * from testtable with (NOLOCK)

reveals a truncated DateField: "2009-04-03 15:41:27.377". The last millisecond is gone.

The Root of the Problem

The datetime data type's inherent limitation is the cause. SQL Server's datetime only supports time precision to approximately 1/300th of a second (0, 3, or 7 milliseconds). Values outside these increments are rounded down. This explains the missing millisecond in our example.

Achieving Millisecond Precision

To maintain millisecond accuracy, alternative methods are necessary. There isn't a single perfect solution, but two common approaches are:

1. Numeric Fields: Store the timestamp as a numeric value (e.g., representing milliseconds since the epoch). This requires custom code to convert to and from a human-readable format.

2. String Representation: Store the timestamp as a string using a consistent format (e.g., 'YYYY-MM-DD HH:mm:ss.fff'). Optionally, include an approximate datetime field for faster querying.

Both methods add complexity. Carefully evaluate the need for millisecond precision against the added design and maintenance overhead.

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