Dubbed Sender Time Zone, the feature, which can be turned on in the Labs tab in Gmail, displays a green phone icon next to a contact in another time zone who's probably awake and reachable. A red phone icon will be displayed next to those who could be sleeping or out of the office. According to a post on Google's Gmail blog, the green icon will be displayed between 9 a.m. and 6 p.m. in the sender's local time zone.
The add-on uses Gmail's message headers to determine time zones. If time zone information isn't readily available in the message header, neither the green nor the red icons will be displayed.
The Sender Time Zone add-on is available now in Gmail Labs.