To generate the changelog reports showing what has changed in the last NN days, you should use the "range" type.
The following configuration will generate reports that covers SCM activities from the last 30 days. Please note that the range is inclusive.
<project>
...
<reporting>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-changelog-plugin</artifactId>
<version>2.3</version>
<configuration>
<type>range</type>
<range>30</range>
</configuration>
</plugin>
</plugins>
</reporting>
...
</project>