What may have happened is that at some point your clock has been set incorrectly when a logrotate has been run, or another error of some sort. The file /var/lib/logrotate.status keeps a record of when files have been rotated by logrotate. Have a look at what line 21 is in the file,
eg. cat -n /var/lib/logrotate.status | grep ' 21'
and if its inconsequential try removing that line, else tweak the dates if that was the problem, might not be though.
And then theres the quick way
eg. rm -f /var/lib/logrotate ;logrotate -f /etc/logrotate.conf
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Damien