RelativeTimestamps is a script to add human readable relative times ("5 minutes ago", "3 hours ago" etc.) after timestamps on watchlist, history, contribution and talk pages, noticeboards etc. Anywhere it finds any appropriately tagged timestamp at least.
New - Sub 1 hour relative times update themselves automatically every minute so they stay up to date while you are reading through a long thread/page.
In your preferences, go to Preferences→ Gadgets. At the bottom of the "Advanced" section, click the "Install scripts without having to manually edit JavaScript files (documentation)" checkbox, then click the "Save" button.
Return to this page and click the blue "Install" button.
Alternatively, you can manually install it by adding the following code to your common.js file:
