Why Do Milliseconds Disappear When Inserting datetime Values into SQL Server?

SQL Server's datetime Precision: The Millisecond Mystery

When inserting a datetime value with milliseconds into a SQL Server database (e.g., 2009-04-03 15:41:27.378), you might notice a seemingly lost millisecond (e.g., it becomes 2009-04-03 15:41:27.377). This isn't a bug; it's a consequence of how SQL Server handles datetime precision.

How SQL Server Stores Time Values

SQL Server's datetime data type has a limitation: it only stores time values with an accuracy of approximately 1/300th of a second. This means that milliseconds are not stored precisely; they are rounded to one of three possible values: 0, 3, or 7.

The Rounding Process

Because the inserted value (27.378 milliseconds) falls between these supported values, SQL Server rounds it down to the nearest permissible millisecond (27.377). This explains the apparent discrepancy.

Maintaining Millisecond Accuracy: Workarounds

To preserve millisecond precision, consider these alternatives:

• Numeric Storage: Store the datetime value as a numeric type (e.g., bigint or decimal) representing the number of milliseconds since a specific epoch. Convert this to a datetime value when required for display or other operations.

• String-Based Storage: Store the datetime value as a string using a consistent format (like yyyy-MM-dd HH:mm:ss.fff). This preserves the full millisecond value, but requires conversion for date/time calculations.

• Hybrid Approach: Use a datetime column for approximate date/time storage (for performance reasons) and a separate string or numeric column to store the exact millisecond value.

These approaches ensure that your millisecond data remains intact, albeit with a slight increase in complexity.

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