from selenium import webdriver
from selenium.webdriver.firefox.options import Options
browser = webdriver.Firefox()
browser.get('https://bookings.doc.govt.nz/Saturn/Facilities/SearchViewGW.aspx')

el = browser.find_element_by_id('mainContent_homeContent_txtArrivalDate')
el.get_property('value')
el.clear()
el.send_keys("21122018")