Due to significant code changes in the Joomla! core, the Visforms field type date, as implemented in Visforms up to version 3.8.21, is broken.
In the field configuration, setting the selected date format for "Initial value" for the date field does not work. Further on the layout of the calendar, that is used for setting this "Initial value" looks broken.
In order to fix the problem you have to update your Visforms installation to the latest Visforms versions (currentliy 3.8.22), which contains the required fixes for this bug. The update should be displayed in the update manager of your website. It is compatible with older Joomla! versions.