Date Modified Formula

The [Date Modified] will only be updated if there is a change to the record.  So the 300 records will not have their [Date Modified] field be updated.

If you were able to include a field in your import such as today's date then any records which were not in the most recent import would have an old date. I mention that because I have a use case where I receive an automated feed of active customers from another system and I need to know when to remove a record. So what I did is arrange for the source import to include the current date. That way every record which was matching one of my current records would be updated and in my data file the records which were not updated start to look suspicious like they have dropped off the data feed.