I’m having the same issue.
My site date setting is d/m/Y (the preview for this is 13/05/2021)
Site time setting (if impacting) is H:I
WP ERP Date format is dd/mm/yyyy
Every time I try to add an employee, it throws the “Please select a valid employee hire date” error
The ‘add new employee’ date field seems to be using the mm/dd/yyyy format, despite the setting for the plugin being dd/mm/yyyy