This page uses the
Pacific/Honolulu zones as examples of how to infer
times of day from the tz database
source files. It might be helpful, but not absolutely necessary,
for the reader to have already downloaded the
latest release of the database and become familiar with the basic layout
of the data files. The format is explained in the “man
page” for the zic compiler,
We’ll begin by talking about the rules for changing between standard and daylight saving time since we’ll need that information when we talk about the zones.
First, let’s consider the special daylight saving time rules
for Chicago (from the
northamerica file in
|From the Source File|
|Reformatted a Bit|
|1920 only||June 13th||02:00 local||go to daylight saving time|
|1920||1921||last Sunday||in October||return to standard time|
|1921 only||in March||go to daylight saving time|
|1954||in September||return to standard time|
TO columns, respectively, specify the
first and last calendar years defining a contiguous range over which a specific
Rule line is to apply. The keyword
only can be used in the
TO field to repeat the value of the
FROM field in the
event that a rule should only apply to a single year. Often, the keyword
max is used to extend a rule’s application into the
indefinite future; it is a platform-agnostic stand-in for the largest
The next column,
-, is reserved; for compatibility with earlier
releases, it always contains a hyphen, which acts as a kind of null value.
Prior to the 2020b release, it was called the
TYPE field, though
it was never used in the main data. An obsolescent supplementary file used the
field as a proof-of-concept to allow
zic to apply a given Rule
line only to certain “types” of years within the specified range as
dictated by the output of a separate script, such as: only years which would
have a US presidential election, or only years which wouldn’t.
SAVE column contains the local (wall clock) offset from
local standard time.
This is usually either zero for standard time or one hour for daylight
saving time; but there’s no reason, in principle, why it can’t
take on other values.
LETTER (sometimes called
column can contain a variable
part of the usual abbreviation of the time zone’s name, or it can just
be a hyphen if there’s no variable part. For example, the abbreviation
used in the central time zone will be either “CST” or
“CDT”. The variable part is ‘S’ or ‘D’;
and, sure enough, that’s just what we find in
Chicago rules. More about this when we talk about
One important thing to notice is that “Rule” lines want at once to be both transitions and steady states:
LETTERcolumns contain state that exists between transitions. More about this when we talk about the US rules.
In the example above, the transition to daylight saving time happened on the 13th of June in 1920, and on the last Sunday in March in 1921; but the return to standard time happened on the last Sunday in October in both of those years. Similarly, the rule for changing to daylight saving time was the same from 1922 to 1966; but the rule for returning to standard time changed in 1955. Got it?
OK, now for the somewhat more interesting “US” rules:
|From the Source File|
|Reformatted a Bit|
|1918||1919||last Sunday||in March||02:00 local||go to daylight saving time|
|in October||return to standard time|
|1942 only||February 9th||go to “war time”|
|1945 only||August 14th||23:00 UT||
rename “war time” to “peace
time;” clocks don’t change
|September 30th||02:00 local||return to standard time|
|1967||2006||last Sunday||in October|
|1973||in April||go to daylight saving time|
|1974 only||January 6th|
|1975 only||February 23rd|
|1976||1986||last Sunday||in April|
|2007||present||second Sunday in March|
|first Sunday in November||return to standard time|
There are two interesting things to note here.
First, the time that something happens (in the
column) is not necessarily the local (wall clock) time. The time can be
suffixed with ‘s’ (for “standard”) to mean
local standard time, different from local (wall clock) time when observing
daylight saving time; or it can be suffixed with ‘g’,
‘u’, or ‘z’, all three of which mean the
standard time at the
‘g’ stands for “GMT”;
‘u’ stands for “UT” or “UTC”
(whichever was official at the time); ‘z’ stands for the
nautical time zone
Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’).
The time can also be suffixed with ‘w’ meaning local (wall
clock) time; but it usually isn’t because that’s the
Second, the day in the
ON column, in addition to
lastSun” or a particular day of the month,
can have the form, “
Sun<=x,” where x is a day
of the month. For example, “
“the first Sunday on or after the eighth of the month,” in
other words, the second Sunday of the month. Furthermore, although
there are no examples above, the weekday needn’t be
Sun” in either form, but can be the usual
three-character English abbreviation for any day of the week.
And the US rules give us more examples of a couple of things already mentioned:
LETTERcolumns contain steady state, not transitions. Consider, for example, the transition from “war time” to “peace time” that happened on August 14, 1945. The “1:00” in the
SAVEcolumn is not an instruction to advance the clock an hour. It means that clocks should be one hour ahead of standard time, which they already are because of the previous rule, so there should be no change.
OK, now let’s look at a Zone record:
|From the Source File|
from Prime Meridian
|Abbreviation(s)||Ending at Local Time|
|−6:00:00||US rules||CST or CDT||1920-01-01||00:00:00|
|−6:00:00||Chicago rules||CST or CDT||1942-01-01||00:00:00|
|US rules||CST, CWT or CPT||1946-01-01|
|Chicago rules||CST or CDT||1967-01-01|
There are a couple of interesting differences between Zones and Rules.
First, and somewhat trivially, whereas Rules are considered to
contain one or more records, a Zone is considered to be a single
record with zero or more continuation lines. Thus, the keyword,
Zone,” and the zone name are not
repeated. The last line is the one without anything in
Second, and more fundamentally, each line of a Zone represents a
steady state, not a transition between states. The state exists from
the date and time in the previous line’s
column up to the date and time in the current
[UNTIL] column. In other words, the date and
time in the
[UNTIL] column is the instant that separates
this state from the next. Where that would be ambiguous because
we’re setting our clocks back, the
specifies the first occurrence of the instant. The state specified by
the last line, the one without anything in the
column, continues to the present.
The first line typically specifies the mean solar time observed
before the introduction of standard time. Since there’s no line before
that, it has no beginning.
8-) For some places near the International
Date Line, the first two lines will show solar times
differing by 24 hours; this corresponds to a movement of the Date
Line. For example:
#Zone NAME STDOFF RULES FORMAT [UNTIL] Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 -8:57:41 - LMT ...
When Alaska was purchased from Russia in 1867, the Date Line moved
from the Alaska/Canada border to the Bering Strait; and the time in
Alaska was then 24 hours earlier than it had
<aside>(6 October in the Julian calendar,
which Russia was still using then for religious reasons, was followed
by a second instance of the same day with a different name, 18
October in the Gregorian calendar. Isn’t civil time
The abbreviation, “LMT” stands for “local mean time”, which is an invention of the tz database and was probably never actually used during the period. Furthermore, the value is almost certainly wrong except in the archetypal place after which the zone is named. (The tz database usually doesn’t provide a separate Zone record for places where nothing significant happened after 1970.)
RULES column tells us whether daylight saving time is being observed:
An example of a specific amount of time is:
#Zone NAME STDOFF RULES FORMAT [UNTIL] Zone Pacific/Honolulu ... 1933 Apr 30 2:00 -10:30 1:00 HDT 1933 May 21 12:00 ...
Hawaii tried daylight saving time for three weeks in 1933 and
decided they didn’t like it.
8-) Note that
STDOFF column always contains the standard time
offset, so the local (wall clock) time during this period was GMT −
10:30 + 1:00 = GMT − 9:30.
FORMAT column specifies the usual abbreviation of
the time zone name. It can have one of three forms:
+”, or “
-”, in which case that’s the abbreviation
/’), in which case the first string is the abbreviation for the standard time name and the second string is the abbreviation for the daylight saving time name
%s,” in which case the “
%s” will be replaced by the text in the appropriate Rule’s
The last two make sense only if there’s a named rule in effect.
An example of a slash is:
#Zone NAME STDOFF RULES FORMAT [UNTIL] Zone Europe/London ... 1996 0:00 EU GMT/BST
The current time in the UK is called either Greenwich mean time or British summer time.
One wrinkle, not fully explained in
zic.8.txt, is what
happens when switching to a named rule. To what values should
LETTER data be initialized?
LETTERdata from the most recent.
SAVEzero), and use the
LETTERdata from the earliest transition with a
And three last things about the
Pacific/Honolulu(shown below) gives “HST” for “Hawaii standard time” even though the legal name for that time zone is “Hawaii-Aleutian standard time.” This author has read that there are also some places in Australia where popular time zone names differ from the legal ones.
"%Z"format specifier to C’s
strftimefunction in the “C” locale.
+07for 7 hours ahead of Greenwich. By convention,
-00is used in a zone while uninhabited, where the offset is zero but in some sense the true offset is undefined.
As a final example, here’s the complete history for Hawaii:
|Relevant Excerpts from the US Rules|
|The Zone Record|
|What We Infer|
|Time Zone||Ending at Local Time|
|−10:31:26||—||LMT||local mean time||1896-01-13||12:00|
|−10:30||+0:01:26||HST||Hawaii standard time||1933-04-30||02:00|
|−9:30||+1:00||HDT||Hawaii daylight time||1933-05-21||12:00|
|−10:30¹||−1:00¹||HST¹||Hawaii standard time||1942-02-09||02:00|
|−9:30||+1:00||HWT||Hawaii war time||1945-08-14||13:30²|
|0||HPT||Hawaii peace time||1945-09-30||02:00|
|−10:30||−1:00||HST||Hawaii standard time||1947-06-08|
|¹Switching to US rules…most recent transition (in 1919) was to standard time|
|²23:00 UT + (−9:30) = 13:30 local|
|³Since 1947–06–08T12:30Z, the civil time in Hawaii has been UT/UTC − 10:00 year-round.|
There will be a short quiz later.