If someone would like to have a time based rolling policy with a size based triggering policy, than the user will get a NullPointerException,:
at ch.qos.logback.core.rolling.helper.Compress.GZCompress(Compress.java:130)
at ch.qos.logback.core.rolling.TimeBasedRollingPolicy.rollover(TimeBasedRollingPolicy.java:142)
at ch.qos.logback.core.rolling.RollingFileAppender.rollover(RollingFileAppender.java:91)
at ch.qos.logback.core.rolling.RollingFileAppender.subAppend(RollingFileAppender.java:116)
at ch.qos.logback.core.WriterAppender.append(WriterAppender.java:124)
at ch.qos.logback.core.AppenderBase.doAppend(AppenderBase.java:81)
at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:51)
at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:282)
at ch.qos.logback.classic.Logger.callAppenders(Logger.java:266)
at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:487)
at ch.qos.logback.classic.Logger.filterAndLog(Logger.java:479)

The cause of this issue is the isTriggeringEvent method in the TimeBasedRollingPolicy which has a side effect, namely it prepares the file name of the file which is about to rotate. If someone uses a triggering policy other than the same TimeBasedRollingPolicy, than the isTriggeringEvent method will not be called, hence the elapsedPeriodFileName will be null, hence a null pointer exception will be thrown in the start of the compression or while renaming.

Please modify the isTriggeringEvent to not have this side effect + the rollover logic to capture the time information from the current event.
Thanks,
Zoltan