Expires Header
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We are considering adding expires header to our site. If we add this meta tag to expire for a certain date, but we do not make any changes to the site after it expires, can you be penalized for this?
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No. The expires directive is merely there to tell the browser "this content is at least good until date X", so that the browser doesn't need to fetch it if it's got it cached.
So let's say you've got a dynamically-generated page called widgets.php, and it pulls a list of products, latest pricing, availability etc. from the database. Let's say you style the page with an external stylesheet.css, you've got some awesome wacky UI stuff in Javascript in an external wackystuff.js, and your images for navigation on the site are in /img/nav (but product images, which maybe can change more frequently) are in /img/products.
You might NOT set the Expires header on your .php files and folders, as those should be considered to expire the second after the browser fetches them. You might set the Expires header for the CSS file to a week out; same with the JS and the /img/nav folders, as those only change with a major update to the site. You might set the Expires header date for the /img/products folder to maybe an hour or two, so that if a person is flipping back and forth between pages, the browser will use the cached product image for a little while before forcing a refetch of it.
Make sense?