Why does Pitz have different time zone lists for New York and Detroit?
>>> pytz.country_timezones['US']
America/New_York
America/Detroit
America/Kentucky/Louisville
America/Kentucky/Monticello
America/Indiana/Indianapolis
America/Indiana/Vincennes
America/Indiana/Winamac
America/Indiana/Marengo
America/Indiana/Petersburg
America/Indiana/Vevay
America/Chicago
America/Indiana/Tell_City
America/Indiana/Knox
America/Menominee
America/North_Dakota/Center
America/North_Dakota/New_Salem
America/North_Dakota/Beulah
America/Denver
America/Boise
America/Phoenix
America/Los_Angeles
America/Metlakatla
America/Anchorage
America/Juneau
America/Sitka
America/Yakutat
America/Nome
America/Adak
Pacific/Honolulu
From everything I can find on the internet, there is absolutely no difference between New York and Detroit clocks, including DST. And that was just from checking the first two. They have a reason to have both, right?
edit: expanding on my research, all of the listed timezones, which are UTC -5, have the same exact DST information, so now it seems even more redundant. I could delete everything below with one for Eastern Standard Time ... Is there something wrong with that?
(UTC -5:00) New_York (UTC -5:00) Detroit (UTC -5:00) Kentucky/Louisville (UTC -5:00) Kentucky/Monticello (UTC -5:00) Indiana/Indianapolis (UTC -5:00) Indiana/Vincennes (UTC -5:00) Indiana/Winamac (UTC -5:00) Indiana/Marengo (UTC -5:00) Indiana/Petersburg (UTC -5:00) Indiana/Vevay
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Pytz uses the IANA Time Zone Database (also known as the Olson Database). As @brenbarn points out, the timezones in the IANA database reflect "any national region where all local clocks all agree on [Jan 1] 1970. " Which includes not only time but DST changes as well.
According to the database , "Most Michigan have watched DST since 1973, but it was a little late in 1975." So this means that Michigan was in EST in 1970 and daylight saving time was on the east coast. In addition, DST did not start in 1975 in Michigan until April 27, a few weeks after the rest of the Eastern Time Zone.
If you download the most recent files, there should be different files (northamerica, southamerica, africa, etc.) that have user-friendly descriptions of what the different time zones represent and an explanation of any changes / reasons why they are not in a different time zone.
To build its edit, Indiana presents a special case in the United States due to its historical presence on the border line between the two time zones.As of November 2014, Indiana is mostly in the Eastern Time Zone (with the exception of a few counties near Chicago and near Evansville which are in central time), and the entire state observes daylight saving time. But the central / eastern border has moved several times since 1970. In fact, I'm sure most of these time zones happened when a number of countries switched to zones in 2006 and 2007, and then adopted Daylight Saving Time in 2007.
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pytz uses zoneinfo timezone definitions, which seem to define the timezone as "any national region where all local clocks all agree since 1970". I would suspect that historical time zone changes or daylight savings time behavior caused them to have the same time now, but not always. (For example, I think Boise and Denver are also equal now, but they also have different records.)
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