Most webpages have a date somewhere -- look for a copyright symbol or a "last upadated" statement -- but if there really is no date, then put "n.d." (for "no date") in place of the date.
Template:
Author, A. A. (n.d.). Title of webpage. Retrieved from Site Name website: htttp://xxxxxxxx
Example:
Royal Institute of British Architects. (n.d.). Shaping the future: Careers in architecture. Retrieved from http://ww.careersinarchitecture.net
Note that Wikipedia is a special case because it is structured as an online encyclopedia.