I've written a GreaseMonkey userscript that recognizes dates and replaces them with links to Google Calendar. So, suppose someone sends you an invitation for a particular date, it will be transformed to a link to that day in Google Calendar.

Currently the following date formats are supported (more to come, write your requests in the comments!):

English
September 1st (Year will be determined)
September 1st 2009
2007-09-01
2007/09/01

Dutch
1 September
1 September 2007

How to install?

Install GreaseMonkey
Install gDate2Cal (hosted at userscripts.org)