If you receive messages from across the globe, Gmail’s new “Sender Time Zone” feature can help you locate appropriate time to call-in a person. Just turn on under Settings, and you'll see green phone icons next to people who’re readily reachable (between 9:00 am and 6:00 pm in sender's local time zone) and red ones next to those who couldn’t be contacted. Click "show details" and you can see when a message was sent in the sender's time zone as well as what time it is for them now.
“Message headers always include the time sent and often include time zone info too. We use that information to show you these icons. If the time zone isn't included for a given message, this Labs feature won't display anything,” Gmail Blog.