The Javadoc for getTimeStamp/setTimeStamp  in ch/qos/logback/classic/spi/LoggingEvent specifies:

Return the number of elapsed seconds since epoch in UTC.

Set the number of elapsed seconds since epoch in UTC.

while there is a code:this.timeStamp = (epochSecond*1000)+(milliseconds); making timeStamp of millisecond precision.

Also Javadoc conflicts with base interface ILoggingEvent with wording:

Return the number of elapsed milliseconds since epoch.