The Time Stamp is the date that appears after the text "posted by" on your blog posts. It is always a good idea to have your most recent blog post have a recent time stamp as visitors will judge look at that date and relate it to how frequently your website is updated.
Click the CONTENT tab on the primary navigation menu
Click the POST tab from the grey horiztonal menu
You will now see all your existing posts and the author column. You can edit current posts by clicking the pencil to the right-hand column.
Once you have opened the post you want to edit located the "Publishing" field. You can now select a publishing status of Publishing (will publish now), Scheduled (select a future date), and Draft (saved but not published) from the list. If you have selected Publishing or Scheduled you can now specific the date you would like to appear with the post by selecting the calendar icon to the right -OR- typing the date out in the format request.
*This is how you can back-date posts as well.
Once you are finished scroll down and select "Edit Post" in the lower right corner. View your live site to see your changes.