Newly launched: Add events to Google Calendar from Gmail
If you do a lot of scheduling over email, it's now a little bit easier to create events directly from your Gmail. Starting today, dates and times within emails are lightly underlined: click them to schedule that conference call or lunch date without ever leaving Gmail.

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.
This new feature is rolling out over the next week to everyone with the English (US) language. We'll be adding other languages soon, so stay tuned.
When you create an event from an underlined date/time within the message, the above Gmail feature will conveniently link to the original message from within the newly-created event. However, for some reason, if you use the drop-down menu to "Create event", then you do not get any link.