timstamped logs

[John Fairhall]

I have an app using log4net with logging set as follows:

<log4net>
    <appender name="RollingFile" type="log4net.Appender.RollingFileAppender">
        <file value="..\log\OOPCollector.log" />
        <appendToFile value="false" />
        <maximumFileSize value="5MB" />
        <maxSizeRollBackups value="5" />
                        <lockingModel 
type="log4net.Appender.FileAppender+MinimalLock" />

        <layout type="log4net.Layout.PatternLayout">
            <conversionPattern value="%date %5level [%thread] %logger - 
%message%newline" />
        </layout>
    </appender>
</log4net>

The app generally generates these logs;
OOPCollector.log
OOPCollector.log.1
OOPCollector.log.2
OOPCollector.log.3
OOPCollector.log.4
OOPCollector.log.5

But sometimes we see logs with a timestamp, such as

OOPCollector.log.2009-09-18
OOPCollector.log.2009-09-18.1
OOPCollector.log.2009-09-18.2
OOPCollector.log.2009-09-18.3
OOPCollector.log.2009-09-18.4
OOPCollector.log.2009-09-18.5

Under what circumstances are these timestamped logs being created?