I literally do this with google all the time though (my girlfriend like us to celebrate N * 100 anniversaries). Every hundred days or so I go to google and type "500 days after [date]" or something to that effect and it works. It was the first thing I tried, it works reliably every time, and it's google providing the result in the results page, which is better than a link to a website that would do it for me.

What query did you do on google that didn't give you so good results?