elem = driver.find_element_by_xpath(
    './/input[@id="ita_form_date_DateTextBox_0"]/following-sibling::input[@type="hidden"]'
    )
value = driver.execute_script('return arguments[0].value;', elem)
print('Before update, hidden input value = {}'.format(value))
driver.execute_script(
    """
    var elem = arguments[0];
    var value = arguments[1];
    elem.value = value;
"""
    , elem, '2013-11-26')
value = driver.execute_script('return arguments[0].value;', elem)
print('After update, hidden input value = {}'.format(value))
Selenium (Python): How to insert value on a hidden input?