Gmail lab provides many good features that you can enable at any time. Features such as multiple inboxes, undo sent message and mail goggle are very useful. Yesterday they added a new feature in Gmail Labs called Sender Time Zone.
Many times it happens to us that when we receive email from friends or others, we don’t know the time zone of sender. We want to call the person but we are not sure whether its mid night there or not. If sender has enabled location info gmail feature, then probably you can find out the time from the location information available in the email. In such cases, Sender Time Zone feature will be useful. It makes sure that its right time to call the sender or not.
Turn it on from the Labs tab under Settings, and you'll see green phone icons next to people who are probably awake and readily reachable (if it's between 9:00 am and 6:00 pm in the sender's local time zone) and red ones next to those who could be sleeping or out of the office:
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.