This option sets the default date format the calendar icon will use on a date widget that does not have Date formatting selected. For example, when Character or Number formatting are selected.
Example:
The following line sets the default date format to USA with 4 digit year
pui.defaultDateFormat="MM/DD/YYYY";
If this option is not specified it will default to "MM/DD/YY"
Where do you save this configuration option? Here.