Hi,
Basically all date formats appearing in the basic tab of Events/category custom fields; as I can see it so far, start/end date and datetime
Stangely enough, for Group categories, the create date custom field does not have such a basic setup tab.
That being said, there are probalbly other possibilities I don't know about yet.
To add to this, in ES Event back-end, Events creation dates are formatted as YYYY-MM-dd even though the standard for ES fields is dd-MM-YYYY.
Hopefully, the tweak is not even more elaborated than the setup.
Thanks.