This is the documentation for ConfiForms Server/Data Center appHowever, this might also work for ConfiForms cloud and in most cases it does. But please see this page to understand the differences between server and cloud versions of the ConfiForms app.
This is something we hear occasionally from our customers.
Basically the answer is simple: ConfiForms uses the global format configured in your Confluence installation for date and date time.
We worked hard to make Date and Datetime pickers to be as localised as possible and to match the formatting and localization settings you have configured.
The global settings for Date and Datetime could be configured through General configuration menu item in Confluence administration