# In this transcript, my comments are marked with leading '#'.
#
# Start with no TZ setting in the environment, which means to
# rely on /etc/localtime, which on my platform is America/Los_Angeles.
$ echo ${TZ-'TZ is unset'}
TZ is unset
$ ls -l /etc/localtime
lrwxrwxrwx. 1 root root 41 May  3 08:15 /etc/localtime -> ../usr/share/zoneinfo/America/Los_Angeles
$ jshell
|  Welcome to JShell -- Version 12.0.1
|  For an introduction type: /help intro

jshell> var wallzone = java.time.ZoneId.systemDefault()
var wallzone = java.time.ZoneId.systemDefault()
wallzone ==> America/Los_Angeles

jshell> java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
$2 ==> 2019-05-31T00:00-07:00[America/Los_Angeles]
# correct

jshell> java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
$3 ==> 2019-01-01T00:00-08:00[America/Los_Angeles]
# correct

jshell>
#
#
# OK, now use the POSIX TZ setting that is equivalent to America/Los_Angeles
# for timestamps from 2007 on.
$ tail -1 /usr/share/zoneinfo/America/Los_Angeles
PST8PDT,M3.2.0,M11.1.0
$ export TZ=PST8PDT,M3.2.0,M11.1.0
$ jshell
|  Welcome to JShell -- Version 12.0.1
|  For an introduction type: /help intro

jshell> var wallzone = java.time.ZoneId.systemDefault()
var wallzone = java.time.ZoneId.systemDefault()
wallzone ==> GMT-08:00

jshell> java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
$2 ==> 2019-05-31T00:00-08:00[GMT-08:00]
# This is wrong: the UTC offset on May 31 is -07, not -08.

jshell> java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
$3 ==> 2019-01-01T00:00-08:00[GMT-08:00]
# correct

jshell>
#
#
# OK, now try Europe/Dublin, a zone with negative DST in winter.
$ export TZ=Europe/Dublin
$ jshell
|  Welcome to JShell -- Version 12.0.1
|  For an introduction type: /help intro

jshell> var wallzone = java.time.ZoneId.systemDefault()
var wallzone = java.time.ZoneId.systemDefault()
wallzone ==> Europe/Dublin

jshell> java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
$2 ==> 2019-05-31T00:00+01:00[Europe/Dublin]
# correct

jshell> java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
$3 ==> 2019-01-01T00:00Z[Europe/Dublin]
# correct

jshell>
#
#
# OK
$ tail -1 /usr/share/zoneinfo/Europe/Dublin
IST-1GMT0,M10.5.0,M3.5.0/1
$ export TZ=IST-1GMT0,M10.5.0,M3.5.0/1
$ jshell
|  Welcome to JShell -- Version 12.0.1
|  For an introduction type: /help intro

jshell> var wallzone = java.time.ZoneId.systemDefault()
var wallzone = java.time.ZoneId.systemDefault()
wallzone ==> GMT+01:00

jshell> java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,5,31,0,0,0,0,wallzone)
$2 ==> 2019-05-31T00:00+01:00[GMT+01:00]
# correct

jshell> java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
java.time.ZonedDateTime.of(2019,1,1,0,0,0,0,wallzone)
$3 ==> 2019-01-01T00:00+01:00[GMT+01:00]
# This is wrong; the UTC offset on January 1 is +00, not +01.

jshell>
$