from selenium import webdriver
driver = webdriver.Firefox()
driver.get('http://jsfiddle.net/falsetru/mLGnB/show/')
elem = driver.find_element_by_css_selector('div.dijitReset>input[type=hidden]')
driver.execute_script(
    """
    var elem = arguments[0];
    var value = arguments[1];
    elem.value = value;
"""
    , elem, '2013-11-26')
Selenium (Python): How to insert value on a hidden input?