most docs don't have both kinds of date_modified_keys (`date_modified` and `date_modified`). Before I had it just always set to None if the date (`case_dict.get(date_modified_key)`) isn't valid. but that would create an extra key=>value pair for the date_modified_key that didn't previously exist.

What I could do instead is set the value to None IFF the key already existed in the case_dict. That might be simpler