cat-datepicker value should be a date iso string

[finnfabians]

Currently, at least in "date" mode, the return object is in the American notation mm/dd/yyyy. It should be a string formatted according to https://en.wikipedia.org/wiki/ISO_8601 (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString), and https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-date-time-string-format