PostgreSQL can accept time zone specifications that
are written according to the POSIX standard's rules
for the TZ environment
variable. POSIX time zone specifications are
inadequate to deal with the complexity of real-world time zone history,
but there are sometimes reasons to use them.
A POSIX time zone specification has the form
STDoffset[DST[dstoffset] [ ,rule] ]
(For readability, we show spaces between the fields, but spaces should not be used in practice.) The fields are:
STD is the zone abbreviation to be used
for standard time.
offset is the zone's standard-time offset
from UTC.
DST is the zone abbreviation to be used
for daylight-savings time. If this field and the following ones are
omitted, the zone uses a fixed UTC offset with no daylight-savings
rule.
dstoffset is the daylight-savings offset
from UTC. This field is typically omitted, since it defaults to one
hour less than the standard-time offset,
which is usually the right thing.
rule defines the rule for when daylight
savings is in effect, as described below.
In this syntax, a zone abbreviation can be a string of letters, such
as EST, or an arbitrary string surrounded by angle
brackets, such as <UTC-05>.
Note that the zone abbreviations given here are only used for output,
and even then only in some timestamp output formats. The zone
abbreviations recognized in timestamp input are determined as explained
in Section B.4.
The offset fields specify the hours, and optionally minutes and seconds,
difference from UTC. They have the format
hh[:mm[:ss]]
optionally with a leading sign (+
or -). The positive sign is used for
zones west of Greenwich. (Note that this is the
opposite of the ISO-8601 sign convention used elsewhere in
PostgreSQL.) hh can have
one or two digits; mm
and ss (if used) must have two.
The daylight-savings transition rule has the
format
dstdate[/dsttime],stddate[/stdtime]
(As before, spaces should not be included in practice.)
The dstdate
and dsttime fields define when daylight-savings
time starts, while stddate
and stdtime define when standard time
starts. (In some cases, notably in zones south of the equator, the
former might be later in the year than the latter.) The date fields
have one of these formats:
nA plain integer denotes a day of the year, counting from zero to 364, or to 365 in leap years.
Jn
In this form, n counts from 1 to 365,
and February 29 is not counted even if it is present. (Thus, a
transition occurring on February 29 could not be specified this
way. However, days after February have the same numbers whether
it's a leap year or not, so that this form is usually more useful
than the plain-integer form for transitions on fixed dates.)
Mm.n.d
This form specifies a transition that always happens during the same
month and on the same day of the week. m
identifies the month, from 1 to 12. n
specifies the n'th occurrence of the
weekday identified by d.
n is a number between 1 and 4, or 5
meaning the last occurrence of that weekday in the month (which
could be the fourth or the fifth). d is
a number between 0 and 6, with 0 indicating Sunday.
For example, M3.2.0 means “the second
Sunday in March”.
The M format is sufficient to describe many common
daylight-savings transition laws. But note that none of these variants
can deal with daylight-savings law changes, so in practice the
historical data stored for named time zones (in the IANA time zone
database) is necessary to interpret past time stamps correctly.
The time fields in a transition rule have the same format as the offset
fields described previously, except that they cannot contain signs.
They define the current local time at which the change to the other
time occurs. If omitted, they default to 02:00:00.
If a daylight-savings abbreviation is given but the
transition rule field is omitted,
PostgreSQL attempts to determine the
transition times by consulting the posixrules file
in the IANA time zone database. This file has the same format as a
full time zone entry, but only its transition timing rules are used,
not its UTC offsets. Typically, this file has the same contents as the
US/Eastern file, so that POSIX-style time zone
specifications follow USA daylight-savings rules. If needed, you can
adjust this behavior by replacing the posixrules
file.
The facility to consult a posixrules file has
been deprecated by IANA, and it is likely to go away in the future.
One bug in this feature, which is unlikely to be fixed before it
disappears, is that it fails to apply DST rules to dates after 2038.
If the posixrules file is not present,
the fallback behavior is to use the
rule M3.2.0,M11.1.0, which corresponds to USA
practice as of 2020 (that is, spring forward on the second Sunday of
March, fall back on the first Sunday of November, both transitions
occurring at 2AM prevailing time).
As an example, CET-1CEST,M3.5.0,M10.5.0/3 describes
current (as of 2020) timekeeping practice in Paris. This specification
says that standard time has the abbreviation CET and
is one hour ahead (east) of UTC; daylight savings time has the
abbreviation CEST and is implicitly two hours ahead
of UTC; daylight savings time begins on the last Sunday in March at 2AM
CET and ends on the last Sunday in October at 3AM CEST.
The four timezone names EST5EDT,
CST6CDT, MST7MDT,
and PST8PDT look like they are POSIX zone
specifications. However, they actually are treated as named time zones
because (for historical reasons) there are files by those names in the
IANA time zone database. The practical implication of this is that
these zone names will produce valid historical USA daylight-savings
transitions, even when a plain POSIX specification would not due to
lack of a suitable posixrules file.
One should be wary that it is easy to misspell a POSIX-style time zone
specification, since there is no check on the reasonableness of the
zone abbreviation(s). For example, SET TIMEZONE TO
FOOBAR0 will work, leaving the system effectively using a
rather peculiar abbreviation for UTC.