Because we continue to encounter some specialized software vendor which can't tell if there is a problem with the Daylight Saving Time change for their application, the need to test the time adjustment beforehand arise sometimes. In this case, the first thing which comes in mind is to change the system clock, without modifying the timezone. Although this can do the job, it doesn't test the DST adjustment properly, and affect the overall operating environment, system-wide.
To do things cleaner, we can try to modify directly the timezone in use.
This will test the real DST automatic adjustment, while not changing the system
clock (and not impacting other software, or services). Say we want to change
DST for Europe/Paris time zone one day before the official date
for summer 2008. First, obtain the current setting:
# zdump -v `rtc` | \
awk '$6 ~ /'"`date '+%Y'`"'/ && $12 !~ /'"`date '+%H:%M:%S'`"'/ {print $0}'
Europe/Paris Sun Mar 30 00:59:59 2008 UTC = Sun Mar 30 01:59:59 2008 CET isdst=0
Europe/Paris Sun Mar 30 01:00:00 2008 UTC = Sun Mar 30 03:00:00 2008 CEST isdst=1
Europe/Paris Sun Oct 26 00:59:59 2008 UTC = Sun Oct 26 02:59:59 2008 CEST isdst=1
Europe/Paris Sun Oct 26 01:00:00 2008 UTC = Sun Oct 26 02:00:00 2008 CET isdst=0
Then, get the time zone source file for the Europe/Paris
geographic definition and adapt it writing an exception rule for the current
year. Because the current definition for Europe/Paris is based on
the EU rule, our modification will be based on this rule name.
# mkdir -p /tmp/zoneinfo/src # cp /usr/share/lib/zoneinfo/src/europe /tmp/zoneinfo/src # diff /usr/share/lib/zoneinfo/src/europe /tmp/zoneinfo/src/europe 1073a1074,1076 > # Test the DST adjustment one day in advance, for March 2008 only. > # Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S > Rule EU 2008 only - Mar 29 2:00 1:00 S
Last, compile the updated time zone definition, put it in place, and verify the new DST date.
# zic -d /tmp/zoneinfo /tmp/zoneinfo/src/europe
# cp /tmp/zoneinfo/Europe/Paris /usr/share/lib/zoneinfo/Europe
# zdump -v `rtc` | \
awk '$6 ~ /'"`date '+%Y'`"'/ && $12 !~ /'"`date '+%H:%M:%S'`"'/ {print $0}'
Europe/Paris Sat Mar 29 00:59:59 2008 UTC = Sat Mar 29 01:59:59 2008 CET isdst=0
Europe/Paris Sat Mar 29 01:00:00 2008 UTC = Sat Mar 29 03:00:00 2008 CEST isdst=1
Europe/Paris Sun Oct 26 00:59:59 2008 UTC = Sun Oct 26 02:59:59 2008 CEST isdst=1
Europe/Paris Sun Oct 26 01:00:00 2008 UTC = Sun Oct 26 02:00:00 2008 CET isdst=0
Once the fake DST adjustment is validated at the software level, clean things up a little.
# zic /usr/share/lib/zoneinfo/src/europe # rm -r /tmp/zoneinfo
