It may not sound like an SEO problem, however displaying a simple calendar on your site has the potential to send the Googlebot into outer space – well maybe infinite space at least.
The problem arises where your calendar has a next month/previous month option. The Googlebot follows links so it will click on the ‘next month’ link, and the ‘next month’ again, and again – you get the picture. It will never leave your site as it continually follows that link. From and SEO point of view this is real bad news. The Goolgebot doesn’t leave that loop so the rest of your site doesn’t get indexed.
If it wasn’t serious it could be funny. However, one of the side effects is an unnecessary use of bandwidth and a failure to read any other links further down the page. The Googlebot is smart enough to learn that it is going nowhere so it backs off. However, by backing off it may simply stop reading and indexing your site at that point. Whilst Googlebot is smart enough to recognize that problem, there are others that could escape. As the Google blog states:
Another common scenario is websites which provide for filtering a set of search results in many ways. A shopping site might allow for finding clothing items by filtering on category, price, color, brand, style, etc. The number of possible combinations of filters can grow exponentially. This can produce thousands of URLs, all finding some subset of the items sold.
This is a good example of why you need to think carefully about every component of your web page. Everything on the page has the potential to affect your search engine optimization activities. In this case, a simple ‘NoFollow’ added will prevent any problems.
Many criticize the use of ‘NoFollow’ as part of an SEM program, however its judicious use does help the various robots follow the links that need to be followed whilst preventing the following of links that provide no value to either the user or the search engines.