You will probably never need to know about the Olson database, also known as the Zoneinfo or tz database. And were it not for my elmcity project I never would have looked into it. I knew roughly that this bedrock database is a compendium of definitions of the world’s timezones, plus rules for daylight savings transitions (DST), used by many operating systems and programming languages.

I presumed that it was written Unix-style, in some kind of plain-text format, and that’s true. Here, for example, are top-level DST rules for the United States since 1918:

# Rule NAME FROM  TO    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

What I didn’t appreciate, until I finally unzipped and untarred a copy of, is the historical scholarship scribbled in the margins of this remarkable database, or document, or hybrid of the two.

You can see a glimpse of that scholarship in the above example. The most recent two rules define the latest (2007) change to US daylight savings. The spring forward rule says: “On the second Sunday in March, at 2AM, save one hour, and use D to change EST to EDT.” Likewise, on the fast-approaching first Sunday in November, spend one hour and go back to EST.

But look at the rules for Feb 9 1942 and Aug 14 1945. The letters are W and P instead of D and S. And the comments tell us that during that period there were timezones like Eastern War Time (EWT) and Eastern Peace Time (EPT). Arthur David Olson elaborates:

From Arthur David Olson (2000-09-25):

Last night I heard part of a rebroadcast of a 1945 Arch Oboler radio drama. In the introduction, Oboler spoke of “Eastern Peace Time.” An AltaVista search turned up :”When the time is announced over the radio now, it is ‘Eastern Peace Time’ instead of the old familiar ‘Eastern War Time.’ Peace is wonderful.”


Most of this Talmudic scholarship comes from founding contributor Arthur David Olson and editor Paul Eggert, both of whose Wikipedia pages, although referenced from the Zoneinfo page, strangely do not exist.

But the Olson/Eggert commentary is also interspersed with many contributions, like this one about the Mount Washington Observatory.

From Dave Cantor (2004-11-02)

Early this summer I had the occasion to visit the Mount Washington Observatory weather station atop (of course!) Mount Washington [, NH]…. One of the staff members said that the station was on Eastern Standard Time and didn’t change their clocks for Daylight Saving … so that their reports will always have times which are 5 hours behind UTC.


Since Mount Washington has a climate all its own, I guess it makes sense for it to have its own time as well.

Here’s a glimpse of Alaska’s timezone history:

From Paul Eggert (2001-05-30):

Howse writes that Alaska switched from the Julian to the Gregorian calendar, and from east-of-GMT to west-of-GMT days, when the US bought it from Russia. This was on 1867-10-18, a Friday; the previous day was 1867-10-06 Julian, also a Friday. Include only the time zone part of this transition, ignoring the switch from Julian to Gregorian, since we can’t represent the Julian calendar.

As far as we know, none of the exact locations mentioned below were permanently inhabited in 1867 by anyone using either calendar. (Yakutat was colonized by the Russians in 1799, but the settlement was destroyed in 1805 by a Yakutat-kon war party.) However, there were nearby inhabitants in some cases and for our purposes perhaps it’s best to simply use the official transition.


You have to have a sense of humor about this stuff, and Paul Eggert does:

From Paul Eggert (1999-03-31):

Shanks writes that Michigan started using standard time on 1885-09-18, but Howse writes (pp 124-125, referring to Popular Astronomy, 1901-01) that Detroit kept

local time until 1900 when the City Council decreed that clocks should be put back twenty-eight minutes to Central Standard Time. Half the city obeyed, half refused. After considerable debate, the decision was rescinded and the city reverted to Sun time. A derisive offer to erect a sundial in front of the city hall was referred to the Committee on Sewers. Then, in 1905, Central time was adopted by city vote.


This story is too entertaining to be false, so go with Howse over Shanks.


The document is chock full of these sorts of you-can’t-make-this-stuff-up tales:

From Paul Eggert (2001-03-06), following a tip by Markus Kuhn:

Pam Belluck reported in the New York Times (2001-01-31) that the Indiana Legislature is considering a bill to adopt DST statewide. Her article mentioned Vevay, whose post office observes a different
time zone from Danner’s Hardware across the street.


I love this one about the cranky Portuguese prime minister:

Martin Bruckmann (1996-02-29) reports via Peter Ilieve

that Portugal is reverting to 0:00 by not moving its clocks this spring.
The new Prime Minister was fed up with getting up in the dark in the winter.


Of course Gaza could hardly fail to exhibit weirdness:

From Ephraim Silverberg (1997-03-04, 1998-03-16, 1998-12-28, 2000-01-17 and 2000-07-25):

According to the Office of the Secretary General of the Ministry of Interior, there is NO set rule for Daylight-Savings/Standard time changes. One thing is entrenched in law, however: that there must be at least 150 days of daylight savings time annually.


The rule names for this zone are poignant too:

# Zone  NAME            GMTOFF  RULES   FORMAT  [UNTIL]
Zone    Asia/Gaza       2:17:52 -       LMT     1900 Oct
                        2:00    Zion    EET     1948 May 15
                        2:00 EgyptAsia  EE%sT   1967 Jun  5
                        2:00    Zion    I%sT    1996
                        2:00    Jordan  EE%sT   1999
                        2:00 Palestine  EE%sT

There’s also some wonderful commentary in the various software libraries that embody the Olson database. Here’s Stuart Bishop on why pytz, the Python implementation, supports almost all of the Olson timezones:

As Saudi Arabia gave up trying to cope with their timezone definition, I see no reason to complicate my code further to cope with them. (I understand the intention was to set sunset to 0:00 local time, the start of the Islamic day. In the best case caused the DST offset to change daily and worst case caused the DST offset to change each instant depending on how you interpreted the ruling.)


It’s all deliciously absurd. And according to Paul Eggert, Ben Franklin is having the last laugh:

From Paul Eggert (2001-03-06):

Daylight Saving Time was first suggested as a joke by Benjamin Franklin in his whimsical essay “An Economical Project for Diminishing the Cost of Light” published in the Journal de Paris (1784-04-26). Not everyone is happy with the results.


So is Olson/Zoneinfo/tz a database or a document? Clearly both. And its synthesis of the two modes is, I would argue, a nice example of literate programming.