"US Eastern Standard Time" vs "Eastern Standard Time" in .NET

Question

In listing all the Id properties of the TimeZoneInfos returned by TimeZoneInfo.GetSystemTimeZones, two versions of EST appear: US Eastern Standard Time and Eastern Standard Time. What's the difference?

I also see both US Mountain Standard Time and Mountain Standard Time, but I'm pretty sure that's because the US version is for Arizona, which doesn't observe DST. I'd assume the regular Mountain Standard Time applies for the rest of the US states in the Mountain time zone. Am I correct?

Helpful link on US time zones: http://www.timetemperature.com/tzus/time_zone.shtml

Wellpreserved

Answer 1

"US Eastern Standard Time" refers to the timezone for the majority of Indiana, which did not observe Daylight Saving Time before 2007. Similarly, "Canada Central Standard Time" is used for Saskatchewan, and "US Mountain Standard Time" for Arizona, which still do not observe DST.

The time zone IDs are assigned in a somewhat illogical way. The "No DST" version mentioned for e.g. the Cayman Islands is called "SA Pacific Standard Time" (SA = South America, and if you look at a timezone map you will see that the -05:00 offset applies to Colombia, Ecuador, and Peru on the west coast of South America)

For another example of timezone IDs "Greenwich Standard Time" which refers to the time zone used in Iceland, which is UTC+00:00 year-round with no DST, whereas "GMT Standard Time" refers to the British timezone which does have DST.

Each time zone corresponds to one of the menu items that you can select from the timezone control panel. This MSDN page shows the correspondence between the text in the menu and the actual timezone ID:

010  Mountain Standard Time         (GMT-07:00) Mountain Time (US and Canada)
013  Mexico Standard Time 2         (GMT-07:00) Chihuahua, La Paz, Mazatlan
015  U.S. Mountain Standard Time    (GMT-07:00) Arizona
020  Central Standard Time          (GMT-06:00) Central Time (US and Canada)
025  Canada Central Standard Time   (GMT-06:00) Saskatchewan
030  Mexico Standard Time           (GMT-06:00) Guadalajara, Mexico City, Monterrey
033  Central America Standard Time  (GMT-06:00) Central America
035  Eastern Standard Time          (GMT-05:00) Eastern Time (US and Canada)
040  U.S. Eastern Standard Time     (GMT-05:00) Indiana (East)
045  S.A. Pacific Standard Time     (GMT-05:00) Bogota, Lima, Quito
085  GMT Standard Time              (GMT) Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London
090  Greenwich Standard Time        (GMT) Casablanca, Monrovia

Bolivia

TLDR; Eastern Standard Time is the correct one – Sap

Note, the link given is only applicable to Windows Embedded 1.1 (2006). There is no static page listing all of the available time zones because they can change as updates are applied. Instead, use tzutil /l on the command line to list the ones installed on the computer. Or, use TimeZoneInfo.GetSystemTimeZones() in .NET. – Une

So the answer is US Eastern Standard Time is only for Indiana? Why didn't they call it Indiana Eastern Standard Time then? Same for Arizona and US Mountain Standard Time? – Campy

Answer 2

The Display Names of those timezones helps explain the confusion

Timezone name	Display name	Observes DST	Offset from UTC in the summer
Eastern Standard Time	(UTC-05:00) Eastern Time (US & Canada)	Yes	-04:00
US Eastern Standard Time	(UTC-05:00) Indiana (East)	Yes	-04:00
Eastern Standard Time (Mexico)	(UTC-05:00) Chetumal	No	-05:00
Mountain Standard Time	(UTC-07:00) Mountain Time (US & Canada)	Yes	-06:00
US Mountain Standard Time	(UTC-07:00) Arizona	No	-07:00
Mountain Standard Time (Mexico)	(UTC-07:00) Chihuahua, La Paz, Mazatlan	Yes	-06:00

• "US Eastern Standard Time" was only for (parts) of Indiana (up until the Energy Policy Act of 2005, which changes the timezones in in Indiana in 2007). Today it is identical to "Eastern Standard Time"
• "US Mountain Standard Time" doesn't observe daylight savings, while the rest of "Mountain Standard Time" does.

Pitarys