This script adds a button to the toolbox, which when clicked gets the history of the talk page, 200 entries at a time, and scans the edit summaries for events likely to be relevant to the {{ArticleHistory}} template. (e.g. any edit summary which contains featured, fac, ga, fl, review, {{any template}}, etc). It outputs the date and edit summary, together with the word (oldid). Clicking on this (in fact anywhere on the line) fetches the revision id of the article on that date.
If you just want to get the oldid corresponding to a given date, click on Click here to enter a date manually and this will bring up a dialog box.