Freebase is young and smart and has many questions; wikipedia, many answers
This program tries to find the date an event happened by searching through its wikipedia article.
Missing date information in freebase is often sitting unambiguously in the first few sentances of a wikipedia article, though this sort of information is too difficult to retrieve accurately without human judgement.

This program was originally a game, but now simply writes its found results to a google spreadsheet for review.

It has found 12 thousand dates
(you must first sign in)

you can use it as an api

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