Script Summary: Press ALT-U to see how long since a page has been refreshed (ex: 01h 15m 32s). If you want to see just minutes and seconds, press SHIFT-ALT-U (ex: 75m 32s). This can be particularly helpful for sites that have a timeout period before downloads, etc.
And thanks to Steven for pointing out that Mac users can use ctrl+alt+u for full display, or ctrl+alt+shift+u for just minutes and seconds.
