If you have a Google Webmaster Tools account you may have noticed the section called ‘Keywords’.
As Google indexes and re-indexes your site, it provides an analysis of the words used in your site and lists them in order of significance.
Common sense dictates that the most used keywords on your site should reflect your main content and intention.
If you’re using Date Archives in your sidebar, the months of the year are being repeated on every single page throughout your site and will likely show up in your top 10 or 20 keywords.
Unless you’re a calendar store you probably don’t want to rank for ‘january’ or ‘october’.
Additionally, if your Date Archives are being indexed by the search engines it’s telling them that the archive page for ‘March 2012′, for instance, has something to do with the actual posts you wrote during that month. Chances are it doesn’t.
A Category Archive or a Tag Archive will bring up a page of posts closely related to a particular category or tag word. Date Archives do not.
Anything you put in your sidebar will be included in the analysis of your site’s keywords and ‘intention’. What are you hoping to rank for?