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Up to now, PG has assumed that any given timezone abbreviation (such as
"EDT") represents a constant GMT offset in the usage of any particular
region; we had a way to configure what that offset was, but not for it
to be changeable over time. But, as with most things horological, this
view of the world is too simplistic: there are numerous regions that have
at one time or another switched to a different GMT offset but kept using
the same timezone abbreviation. Almost the entire Russian Federation did
that a few years ago, and later this month they're going to do it again.
And there are similar examples all over the world.
To cope with this, invent the notion of a "dynamic timezone abbreviation",
which is one that is referenced to a particular underlying timezone
(as defined in the IANA timezone database) and means whatever it currently
means in that zone. For zones that use or have used daylight-savings time,
the standard and DST abbreviations continue to have the property that you
can specify standard or DST time and get that time offset whether or not
DST was theoretically in effect at the time. However, the abbreviations
mean what they meant at the time in question (or most recently before that
time) rather than being absolutely fixed.
The standard abbreviation-list files have been changed to use this behavior
for abbreviations that have actually varied in meaning since 1970. The
old simple-numeric definitions are kept for abbreviations that have not
changed, since they are a bit faster to resolve.
While this is clearly a new feature, it seems necessary to back-patch it
into all active branches, because otherwise use of Russian zone
abbreviations is going to become even more problematic than it already was.
This change supersedes the changes in commit
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| .. | ||
| data | ||
| tznames | ||
| .gitignore | ||
| ialloc.c | ||
| known_abbrevs.txt | ||
| localtime.c | ||
| Makefile | ||
| pgtz.c | ||
| pgtz.h | ||
| private.h | ||
| README | ||
| scheck.c | ||
| strftime.c | ||
| tzfile.h | ||
| zic.c | ||
src/timezone/README Timezone ======== This is a PostgreSQL adapted version of the timezone library from http://www.iana.org/time-zones The source code can be found at: ftp://ftp.iana.org/tz/releases/tzcode*.tar.gz The code is currently synced with release 2010c. There are many cosmetic (and not so cosmetic) differences from the original tzcode library, but diffs in the upstream version should usually be propagated to our version. The data files under data/ are an exact copy of the latest data set from: ftp://ftp.iana.org/tz/releases/tzdata*.tar.gz Since time zone rules change frequently in some parts of the world, we should endeavor to update the data files before each PostgreSQL release. At each update, we should check if time zone offsets have changed. Just search for the current or previous year and see what has changed. Sometimes a country changes its time zone offsets, for example Georgia in 2004. Just grepping in the zic database files for 2004 is enough to spot such a change. Then the files under tznames/ should be updated. When there has been a new release of Windows (probably including Service Packs), the list of matching timezones need to be updated. Run the script in src/tools/win32tzlist.pl on a Windows machine running this new release and apply any new timezones that it detects. Never remove any mappings in case they are removed in Windows, since we still need to match properly on the old version.