It is easy on blogs only if the blog author displays the last modified date. most blogs display only the posted date.Guess, that is what you too show.

document.lastmodified will return the last modifed date only if the page that your browse sends that as part of its http header and that too only if they return it correctly.For eg: you try opening this post in different tabs after a gap of a few seconds.It will show, different lastmodified dates.This implies that wordpress returns a last modifed date that is not correct.it just returns the server datetime that you access the post and not necessarily the last modified datetime.

this can atbest be true only for static pages and not for something dynamic like wordpress posts/pages.