I had problem with getting dates to show up properly in both systems. I searched a bit and found that neither support DST. A bit more search showed that PHP (the language in which both are implemented) has all the functionality to provide DST support. For the developers of WordPress and MediaWiki, supporting DST is simply a matter of performing the right function calls to PHP which in turn relies on the OS for DST support. Basically, neither WordPress nor MediaWiki have to do complicated DST calculations or worry about the various ways various locales deal with it: the OS takes care of all that.
Still, both WordPress and MediaWiki require their users to mess around with manually switching timezone offsets twice a year. Why, oh why do users have to deal with this when it would be so easy to implement an automatic solution?!? Such an omission is mind boggling.