Some dates and times within emails are lightly underlined: click them to schedule an event in your calendar without ever leaving Gmail.

calendar

When you click on one of these underlined dates, you'll be able to preview your schedule for the day and change the title, date or time of the event. Clicking “Add to Calendar” will do exactly that -- add the event to your calendar, and for extra convenience, the calendar event will include a link back to the original email.

via: gmailblog.blogspot.com