Google has introduced a new GMail feature today which allows you to add a new calendar event right from an email. If you open an email with a date or time in it, GMail will lightly underline it. If you click on it, it’ll open a popup box for the Google Calendar and automatically populate some information from the email (as seen above).
Google is slowly rolling this feature out over the next few weeks and it is currently only coming to people in the US.
Source: GMail Blog