V interesting. I've been looking at this too. There were a few deficiencies in the google implementation that bugged me. (1) You have to click on icons to read the info, I think the wikipedia layer should be browsable and ideally include photos. (2) No way to filter to certain articles (e.g. WWII battles) and (3) No date filter - as you've spotted. Tweet me @mbrenig if you want to talk more.