ADDITIONAL SYSTEM INFORMATION :
Full commandline from one test. This one has the explicit parameter to disable PrintGCTimeStamps.

"C:\Program Files\Java\jdk1.8.0_144\bin\java" -server -Xms512m -Xmx512m -Duser.timezone=UTC -XX:NewRatio=3 -XX:SurvivorRatio=4 -XX:TargetSurvivorRatio=90 -XX:MaxTenuringThreshold=8 -XX:+UseConcMarkSweepGC -XX:ConcGCThreads=4 -XX:ParallelGCThreads=4 -XX:+CMSScavengeBeforeRemark -XX:PretenureSizeThreshold=64m -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSInitiatingOccupancyFraction=50 -XX:CMSMaxAbortablePrecleanTime=6000 -XX:+CMSParallelRemarkEnabled -XX:+ParallelRefProcEnabled -XX:-OmitStackTraceInFastThrow -verbose:gc -XX:+PrintHeapAtGC -XX:+PrintGCDetails -XX:-PrintGCTimeStamps -XX:+PrintGCDateStamps -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime "-Xloggc:C:\Users\sheisey\Downloads\solr-7.3.0\server\logs\solr_gc.log" -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=9 -XX:GCLogFileSize=20M -Xss256k -Dsolr.log.dir="C:\Users\sheisey\Downloads\solr-7.3.0\server\logs" -Dlog4j.configuration="file:C:\Users\sheisey\Downloads\solr-7.3.0\server\resources\log4j.properties" -DSTOP.PORT=7983 -DSTOP.KEY=solrrocks -Dsolr.log.muteconsole -Dsolr.solr.home="C:\Users\sheisey\Downloads\solr-7.3.0\server\solr" -Dsolr.install.dir="C:\Users\sheisey\Downloads\solr-7.3.0" -Dsolr.default.confdir="C:\Users\sheisey\Downloads\solr-7.3.0\server\solr\configsets\_default\conf" -Djetty.host=0.0.0.0 -Djetty.port=8983 -Djetty.home="C:\Users\sheisey\Downloads\solr-7.3.0\server" -Djava.io.tmpdir="C:\Users\sheisey\Downloads\solr-7.3.0\server\tmp" -jar start.jar "--module=http" ""

Also tried with 8u172, and tried Solr 5.5.5 with 7u80, 7u_45, and 7u4, with the same behavior.

A DESCRIPTION OF THE PROBLEM :
Running latest Solr version. GC logging on the commandline has PrintGCDateStamps enabled. Want to disable PrintGCTimeStamps so there is only one timestamp in the log. Commandline did have -XX:+PrintGCTimeStamps included, so that was removed. Also tried with -XX:-PrintGCTimeStamps to explicitly disable it. This did not work -- both the absolute date stamp and the relative seconds-since-JVM-start timestamps are in the GC log. I used the JDK, not the JRE, for all these tests. I have only done the testing on Windows, though when I run these things for real, it's on Linux.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run a program with GC logging options that include -XX:+PrintGCDateStamps and do NOT include -XX:+PrintGCTimeStamps. Alternately, use -XX:+PrintGCDateStamps -XX-PrintGCTimeStamps so that the latter is explicitly disabled.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The gc logfile should only contain absolute full date/time information, not seconds-since-JVM-start.

ACTUAL -
Both the full date/time information AND seconds-since-JVM-start are included in every log entry.