The following message requests that $OPT use ISO date/time format:

http://mail.openjdk.java.net/pipermail/verona-dev/2015-December/000323.html

This is the critical portion of the request:

$OPT of JEP 223 is not easily parseable/not a valid ISO date:

java --version
> Java(TM) SE Runtime Environment (build
> 9-ea+99-2015-12-23-183325.javare.4146.nc)

Can you kindly file an issue to make that a proper basic ISO datetime string? Date format would be here yyyyMMdd'T'HHmmss'Z' with timezone Etc/UTC as in DateTimeFormatter#basicDateTimeNoMillis.