Something like below to validate date.

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading || newValue === '') {
        return;
    }
    var isValidDate = validateDate(newValue);
    if (!isValidDate) {
        g_form.showFieldMsg('u_string_date', 'invalid date', 'error');
    }

    function validateDate(str) {
        try {
            var datePattern = g_user_date_format;
            datePattern = datePattern.replaceAll('d', '\\d');
            datePattern = datePattern.replaceAll('y', '\\d');
            datePattern = datePattern.replaceAll('M', '\\d');

            datePattern = new RegExp(datePattern);
            var isDate = datePattern.test(str);
            if (!isDate) {

                return false;
            }

            var nDate = getDateFromFormat(newValue, g_user_date_format);
            if (nDate == 0) {
                return false;
            }
        } catch (e) {
            return false;
        }
        return true;
    }
}