How to Read the tz Database Source Files

by Bill Seymour

This paper is in the public domain.

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 the data subdirectory):

From the Source File
# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER Rule Chicago 1920 only - Jun 13 2:00 1:00 D Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
Reformatted a Bit
From	To	On		At	Action
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
1922	1966		in April
	1954		in September		return to standard time
1955	1966		in October

We’ll basically just ignore the TYPE column. In the 2007j release, the most recent as of this writing, the TYPE column never contains anything but a hyphen, a kind of null value. (From the description in zic.8.txt, this appears to be a mechanism for removing years from a set in some localizable way. It’s used in the file, pacificnew, to determine whether a given year will have a US presidential election; but everything related to that use is commented out.)

The SAVE column contains the 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.

The LETTER (sometimes called LETTER/S) 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 the LETTER column in the Chicago rules. More about this when we talk about “Zone” lines.

One important thing to notice is that “Rule” lines want at once to be both transitions and steady states:

• On the one hand, they represent transitions between standard and daylight saving time; and any number of Rule lines can be in effect during a given period (which will always be a non-empty set of contiguous calendar years).

• On the other hand, the SAVE and LETTER columns 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 13^th 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
# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S Rule US 1918 1919 - Mar lastSun 2:00 1:00 D Rule US 1918 1919 - Oct lastSun 2:00 0 S Rule US 1942 only - Feb 9 2:00 1:00 W # War Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace Rule US 1945 only - Sep 30 2:00 0 S Rule US 1967 2006 - Oct lastSun 2:00 0 S Rule US 1967 1973 - Apr lastSun 2:00 1:00 D Rule US 1974 only - Jan 6 2:00 1:00 D Rule US 1975 only - Feb 23 2:00 1:00 D Rule US 1976 1986 - Apr lastSun 2:00 1:00 D Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D Rule US 2007 max - Mar Sun>=8 2:00 1:00 D Rule US 2007 max - Nov Sun>=1 2:00 0 S
Reformatted a Bit
From	To	On		At	Action
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
1987	2006	first Sunday
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 AT 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 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 prime meridan. ‘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 “wall clock time;” but it usually isn’t because that’s the default.

Second, the day in the ON column, in addition to “lastSun” or a particular day of the month, can have the form, “Sun>=x” or “Sun<=x,” where x is a day of the month. For example, “Sun>=8” means “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:

• The rules for changing to and from daylight saving time are actually different sets of rules; and the two sets can change independently. Consider, for example, that the rule for the return to standard time stayed the same from 1967 to 2006; but the rule for the transition to daylight saving time changed several times in the same period. There can also be periods, 1946 to 1966 for example, when no rule from this group is in effect, and so either no transition happened in those years, or some other rule is in effect (perhaps a state or other more local rule).

• The SAVE and LETTER columns 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 SAVE column 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
# Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 -6:00 US C%sT 1920 -6:00 Chicago C%sT 1936 Mar 1 2:00 -5:00 - EST 1936 Nov 15 2:00 -6:00 Chicago C%sT 1942 -6:00 US C%sT 1946 -6:00 Chicago C%sT 1967 -6:00 US C%sT
Columns Renamed
Standard Offset from Prime Meridian	Daylight Saving Time	Abbreviation(s)	Ending at Local Time
			Date	Time
−5:50:36	not observed	LMT	1883-11-18	12:09:24
−6:00:00	US rules	CST or CDT	1920-01-01	00:00:00
	Chicago rules		1936-03-01	02:00:00
−5:00:00	not observed	EST	1936-11-15
−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
	US rules		—

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 the [UNTIL] column.

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 [UNTIL] column up to the date and time in the current line’s [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 [UNTIL] column specifies the first occurrence of the instant. The state specified by the last line, the one without anything in the [UNTIL] 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		GMTOFF	RULES	FORMAT	[UNTIL]
Zone America/Juneau	 15:02:19 -	LMT	1867 Oct 18
			 -8:57:41 -	LMT	...

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.)

The RULES column tells us whether daylight saving time is being observed:

• A hyphen, a kind of null value, means that we have not set our clocks ahead of standard time.
• An amount of time (usually but not necessarily “1:00” meaning one hour) means that we have set our clocks ahead by that amount.
• Some alphabetic string means that we might have set our clocks ahead; and we need to check the rule the name of which is the given alphabetic string.

# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
Zone Pacific/Honolulu	...			1933 Apr 30 2:00
			-10:30	1:00	HDT	1933 May 21 2:00
			...

The FORMAT column specifies the usual abbreviation of the time zone name. It can have one of four forms:

• the string, “zzz,” which is a kind of null value (don’t ask)
• a single alphabetic string other than “zzz,” in which case that’s the abbreviation
• a pair of strings separated by a slash (‘/’), 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
• a string containing “%s,” in which case the “%s” will be replaced by the text in the appropriate Rule’s LETTER column

An example of a slash is:

# Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
Zone	Europe/London	...			1996
			 0:00	EU	GMT/BST

One wrinkle, not fully explained in zic.8.txt, is what happens when switching to a named rule. To what values should the SAVE and LETTER data be initialized?

• If at least one transition has happened, use the SAVE and LETTER data from the most recent.

• If switching to a named rule before any transition has happened, assume standard time (SAVE zero), and use the LETTER data from the earliest transition with a SAVE of zero.

And two last things about the FORMAT column:

• The tz database gives abbreviations for time zone names in popular usage, which is not necessarily “correct” by law. For example, the last line in Zone 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.

• No attempt is made to localize the abbreviations. They are intended to be the values returned through the "%Z" format specifier to C’s strftime function in the “C” locale.

As a final example, here’s the complete history for Hawaii:

Relevant Excerpts from the US Rules
# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S Rule US 1918 1919 - Oct lastSun 2:00 0 S Rule US 1942 only - Feb 9 2:00 1:00 W # War Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace Rule US 1945 only - Sep 30 2:00 0 S
The Zone Record
# Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 -10:30 - HST 1933 Apr 30 2:00 -10:30 1:00 HDT 1933 May 21 2:00 -10:30 US H%sT 1947 Jun 8 2:00 -10:00 - HST
What We Infer
Wall-Clock Offset from Prime Meridian	Adjust Clocks	Time Zone		Ending at Local Time
		Abbrv.	Name	Date	Time
−10:31:26	—	LMT	local mean time	1900-01-01	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
−10:30¹	−1:00¹	HST¹	Hawaii standard time	1942-02-09
−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
−10:00³	+0:30³			—
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. 8-)