Late Wednesday night Gmail Labs introduced a new feature called sender time zone. The feature adds a tiny red or green icon (the color depends on the sender’s local time) to all incoming email so you can tell at a glance whether local time for the sender falls inside or outside of the workday.
In order to turn this new feature on, you’ll need to use the settings menu within Gmail. Then navigate to the Labs tab.
Turn on the sender time zone feature and let Gmail restart.
I’ve found this to be an especially helpful feature for knowing whether I might expect a reply from someone that I’m emailing. With sender time zone enabled each email that lands in your in box has either a red icon or a green icon depending on whether the sender’s time is between 9am and 6pm (green) or outside those hours.
Unfortunately there’s no way to set your working hours so you’ll have to make due with the 9am to 6pm rules for determining whether the icon is red or green.