Extension:TimeMachine

The TimeMachine extension allows you to browse a wiki as it was at any given date. By going to Special:TimeMachine, you can set a date and from then on whenever you visit a page, you will see the first revision previous to that date. By visiting Special:TimeMachine again, or emptying your cookies, you can return to browsing the wiki as always.

The design and implementation of TimeMachine are described here.