Thanks for the advice, John.  The inserts are being done by a 3rd-party application that I have no control over, so hence using the trigger.

Unfortunately the table has no indexes, so to join INSERTED with the main table in an insert trigger I believe would cause a huge performance hit (the main table will have hundred of thousands of rows, and has no distinguishable unique fields to use for a primary key)

Most likely candidate for an index is the DateTime field called DateTimeLocal. It is not unique, but the most rows with identical values I've seen so far is less than 100, so a non-unique index on this field could work.

Wish me luck!