If nothing is logged during an interval, the TimedRotatingFileHandler
will give bad names to future log files.

The enclosed example program sets up a logger that rotates the log every
second.  It then logs a few messages with sleep of 1, 2, 4, 1 and 1
seconds between the messages.  The log files will have names that
increase with one second per log file, but the content for the last file
will be generated a different second.

An example run produced the message

  2008-03-17 09:16:06: 1 sec later

in a log file named badlogdir/logfile.2008-03-17_09-16-02.

This problem was likely introduced in revision 42066.  The root cause is
that self.rolloverAt is increased by self.interval in doRollover - but
if nothing was logged for a while, it should be increased by a multiple
of self.interval.