Milliseconds to datetime
DATETIME is used to store an instant ("point in time") at a particular granularity (approx 300 milliseconds IIRC).

The value you wish to store is an interval (or is it a duration?) in milliseconds. SQL Server lacks an interval data type.

While both are temporal data they have quite different requirements. I suggest you use an numeric column (e.g. INTEGER) to store the interval and use metadata (e.g. an appropriate name) to convey the fact the granularity is one millisecond.