B.5. POSIX Time Zone Specifications
can accept time zone specifications
that are written according to the
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
dstoffset] [ ,
(For readability, we show spaces between the fields, but spaces should not be used in practice.) The fields are:
STDis the zone abbreviation to be used for standard time.
offsetis the zone's standard-time offset from UTC.
DSTis 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.
dstoffsetis 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.
ruledefines 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
, or an arbitrary string surrounded by angle
brackets, such as
The offset fields specify the hours, and optionally minutes and seconds,
difference from UTC. They have the format
optionally with a leading sign (
). The positive sign is used for
of Greenwich. (Note that this is the
opposite of the ISO-8601 sign convention used elsewhere in
can have one or two digits;
(if used) must have two.
The daylight-savings transition
(As before, spaces should not be included in practice.)
fields define when daylight-savings
time starts, while
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:
A plain integer denotes a day of the year, counting from zero to 364, or to 365 in leap years.
In this form,
ncounts 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.)
This form specifies a transition that always happens during the same month and on the same day of the week.
midentifies the month, from 1 to 12.
n'th occurrence of the weekday identified by
nis 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).
dis a number between 0 and 6, with 0 indicating Sunday. For example,
M3.2.0means " the second Sunday in March " .
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
If a daylight-savings abbreviation is given but the
field is omitted,
the fallback behavior is to use the
, 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). Note that this rule does not
give correct USA transition dates for years before 2007.
As an example,
current (as of 2020) timekeeping practice in Paris. This specification
says that standard time has the abbreviation
is one hour ahead (east) of UTC; daylight savings time has the
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
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.
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
will work, leaving the system effectively using a
rather peculiar abbreviation for UTC.