Fix post date

Instead of changing the reference to the post like in #2367, I prefer to fix the date of the post itself. Note that the link http://jekyllrb.com/news/2013/05/05/jekyll-1-0-0-released/ doesn’t change.

And I changed the time zone to Los Angeles like @parkr suggested in https://github.com/jekyll/jekyll/pull/2367#issuecomment-42783850