Why is setting stringResult = true is not using the specified format to format the chosen date? Even in the sample page http://docs.webix.com/samples/13_form/01_controls/13_datepicker.html clicking any of the “Submit data” buttons show that the string result is always using YYYY-MM-DD hh:mm for date and hh:mm for time, regardless of the specified format.
The thing is that datepicker format is a format of data presentation only, which means that it is used only to show the date in the input field.
The value that the datepicker actually returns is formatted by webix.i18n.parseFormatStr method that uses a global webix.i18n.parseFormat. So you need to change the parseFormat in the currently used locale to change the output value.
It is done with date uniformity in mind, supposing that all dates in the application should have one and the same format, which may simplify their saving to the server side.