See thread: http://forums.atlassian.com/thread.jspa?threadID=5480&tstart=0

Most recently:

What about having it so that you can pass in a time period and a max count.
The time period would be something like 7d4h for 7 days 4 hours (otherwise
assumed to be "forever"), and a max count would be a number of blogs to
show. The idea is very similar, just adapted slightly - see the examples:

{blog-posts:max=5|time=7d}

show up to 5 blog posts in the last week

{blog-posts:time=1m}

show all the blog posts for the last month

{blog-posts:max=10}

show the latest 10 blog posts - regardless of time frame

The original thought behind this was to see if the blog feature could be used for "motd" like applications, such as system status notices. These tend to be only applicable for a short time, hence some form of expiration would be nice.