I was looking at your function and I am sure it can be improved, unfortunately with all the conversions between datetime and character formats and the lack of perspective (we can't see the structure of the table(s) queried nor the data stored).

Using proper data types will definitely help the performance of your routine, and it could probably be rewritten as an iTVF (inline table valued function).

If you would like to see this, please post the DDL for the table(s) involved and some sample data. Please read the first article I reference in my signature block regarding asking for help. It will walk you through the steps on what you need to post and how to post it to get the best possible answers in return.