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Thanks a lot for your comprehensive answer. It's a totally custom site so I will have to hire a developer to do the job.

This would be really hard to pinpoint without seeing the page. As it all depends on what's on your page. For example, if it's a blog post that you just wrote, but put the timestamp at last year by accident, google will see that in the code and use that. Another example could be that somewhere in the text you've written "in december 2009 SEO was a lot different" it could incorrectly pick that up and place it within other snippets.