In an answer of a question on GoogleWebmaster channel on Youtube, MattCutts explains that Google doesn't use URL structure as a PageRank factor. Google doesn't worry very much about how deep a set of directories is. If a page with deep URL structure, and is linked by another page, it still gets pagerank from that page.
On his WordPress blog, MattCutts uses
/postname/ as URL structure instead of
/year/name/month/postname/. It doesn't affect PageRank as explained above. The reason he chose it because it's easy to remember. And the date-based URL structure isn't always bad. When visitors look at this, they know exactly when the posts were created, and might look for a new version if they're old.
So if you have a date-based URL structure, don't worry about its effect to pagerank. You can see Google uses date-based URL structure for Blogger. Keep blogging with good content, it's the most important!
This is the full video of this question and answer: