To resolve this, realize that you are not limited to the choices in the drop down but rather you can type in any value that Microsoft accepts. For instance, we had one customer that wanted to save all PDFs with year/month/day format listed first. So, this customer could just type in "yyyyMMdd" in the date field, and then the rest of the rename field as normal:
Notice how we simply delete the Time Format entry, since the customer didn't want the time showing (but make sure that "Overwrite existing files" is not checked, because you might end up with a lot of filename collisions!).
Also note that the month portion of "yyyyMMdd" is in capital letters. This is to distinguish it from minutes which is a lowercase "m". Microsoft has a complete list of all the formats available starting at https://docs.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings#the-d-custom-format-specifier
This article applies to:
Attachment Save
Rename Attachments
Save As PDF
Save As PDF Pro