The module doesn't seem to be compatible with other core date types besides timestamp?
(I'm posting this as bug report, because that doesn't seem to be documented on the module page)
Try using this with date fields "date" or "datetime", but the "Use SQL to format date" switch simply isn't there.
1. Document on the module page, that this currently only works with "timestamp" date fields
2. Support other date formats, especially "date" and "datetime"
Active
3.0
Code
If anyone else encounters this: My workaround now is to use https://www.drupal.org/project/views_raw_sql → and format the date field with custom SQL and hide the original date field. Afterwards, the output can be used for grouping.