Demo pokazuje użycie różnych opcji dla inputów typu text i date:

input text:

const btn1 = document.querySelector('.demo');
const cal1 = new Calendar(btn1, {
    closeOnSelect : true,
    todayBtnText : 'Today',
    onDateSelect : function(day, month, year) {
        const dayText = ((day + 1) < 10) ? "0" + (day + 1) : day + 1;
        const monthText = ((month + 1) < 10) ? "0" + (month + 1) : month + 1;

        btn1.value = dayText + '/' + monthText + '/' + year;
    }
});
cal1.init();

input date:

const btn2 = document.querySelector('.demo2');
const cal2 = new Calendar(btn2, {
    closeOnSelect : true,
    showTodayBtn : false,
    onDateSelect : function(day, month, year) {
        const dayText = ((day + 1) < 10) ? "0" + (day + 1) : day + 1;
        const monthText = ((month + 1) < 10) ? "0" + (month + 1) : month + 1;

        btn2.value = dayText + '.' + monthText + '.' + year;
    }
});
cal2.init();

input number:

const btn3 = document.querySelector('.demo3');
const cal3 = new Calendar(btn3, {});
cal3.init();