Postbox 3 will turn dates (ex: "next Thursday" and "12/31/2012") inside email messages into links that you can click to add events to your calendar.
On Windows, the default calendar is Google Calendar.
On Mac, the default calendar is Calendar app, and Postbox will add events to the first calendar on your list. Drag and drop the desired calendar to the top of your list, and this can either be your default calendar or another calendar.
To use Google Calendar on the Mac, go to:
Postbox | Preferences | Advanced | Web Services
and check the option "Use Google Calendar for detected dates."
To disable this feature, use the Postbox Config Editor and set the hidden mail.datapane.detectors.dates preference to false.