@ Joost,
Never mind, you're a busy man.
For others out there who might be plagued by this problem:
Yoast's SEO plugins are quite powerful, and after activation you may see changes to meta title/descriptions which were not being picked up by Google serps previously.
This problem is related to what's contained in your theme files.
That annoying date at the start of our meta description was the date our blog was originally started!
Nothing had been changed in header.php, index.php or single.php to make the date start appearing - only activation of Yoast's Video SEO plugin.
Here's how I fixed it:
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Most importantly; I needed to remove the date completely from all pages.
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I didn't want to remove the date entirely from posts - just the serps.
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I did want to remove the date from posts: 6 months after first published date.
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I also wanted to remove "Last Updated" time on Google serps - but have this always displayed on the post.
First step is to hide the time in javascript so Google can no longer read it. This means it will still show on the post, but not serps.
This is the code I now have in my theme's index.php and single.php files:
<p id="author"><strong>Written by:</strong> <?php the_author() ?> <script language="javascript" type="text/javascript">document.write("<?php the_time('F jS, Y') ?>");</script><br><strong>Last updated:</strong> <time datetime="<?php the_modified_time('Y-m-d'); ?>"><?php the_modified_time('F jS, Y'); ?></time></p>
Of course your theme may be using "get_time" or something else similar instead of "the_time".
Second step is to use the 'Date Exclusion' plugin, to stop the published date appearing on posts after 6 months.
Some really useful posts about this:
http://www.wpsquare.com/remove-date-meta-description-search-results/
http://www.dollarshower.com/how-to-remove-date-stamp-from-google-serp-description/
If you are still having problems, you can always go for the "Big Hammer" of funtions.php code, mentioned in that first article.
Why have the "Last Updated" date showing?
This article explains the issues around removing dates fairly well (of course your individual website needs may differ):
http://www.wpbeginner.com/opinion/why-you-should-not-remove-dates-from-your-wordpress-blog-posts/
Google obviously puts a lot of emphasis on dates, but as search engine spiders are not smart enough to differentiate between dates (and pick the right one); removing their ability to read dates is a good idea.
Who wants every page of your website showing the date you first started your blog?
Removing the date completely (for your readers) may not be such a great idea.