Gmail Labs added a very useful feature for those who receive messages from all over the world. "Sender Time Zone" shows a green phone icon if it's appropriate to call the sender and a red phone icon if it's not a good time for calling (the icon is actually an Unicode character). Click on "show details" and you'll also see the local time of the sender.
"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," explains Gmail's blog. Google saves your timezone in your Google Account and it can be changed from this page.
You can also use Google search to find the time in a certain location.