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

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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

timezone .net c#
2 Answers

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.
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