Hi @Rick A. Anderson
According to the official documentation, the Standard edition of SQL Server Reporting Services does not support data-driven report subscriptions. I recommend you use the Developer version.
If you don't want to change the version, then you can try adding "@timestamp" to the file name, like this:
Result:
If the answer is helpful, please click "Accept Answer" and upvote it. If you have any questions, please feel free to let me know.
Best regards,
Aniya