The patterns property (string[]) sets additional date format patterns that are tried when reading the value from the target element.
DayPilot.DatePicker.patterns[]The pattern value is used first. If parsing fails, the values in patterns are tried in the specified order.
const picker = new DayPilot.DatePicker({
target: "start",
patterns: ["M/d/yyyy", "yyyy-MM-dd"],
// ...
});
picker.show();