I think google has some quirks with the last modified header in HTTP. When doing a couple searches where I know they've existed before the listed date (e.g. as

I think google has some quirks with the last modified header in HTTP. When doing a couple searches where I know they've existed before the listed date (e.g. astalavista, jabber ccc, MIT opencourseware etc) it seems that google chimes in with quotations in other sources like pdf files or papers and if the pdf file is from an earlier date, it seems to recorrect the linked URL's last modified HTTP header date. Interesting to observe, thought this was worth mentioning.