与DateTime偏移量混淆 [英] Confused by DateTime offsets

问题描述

我试图了解如何解释GMT偏移量，以便可以在R中使用日期时间对象。例如，假设我有一个类似 2011-04-04 10:45:00 GMT + 10的日期时间

第一季度：我应该将其作为格林威治时间并加上10个小时以获取当地时间吗？还是当地时间，我需要减去10个小时才能获得格林尼治标准时间？我一直都理解是后者。

Q2：为什么R似乎使用前一种解释？例如

  foo<-as.POSIXct（ 2011-04-04 10:45:00，tz = Etc / GMT + 10）
 attr（foo， tzone）<- GMT 
 foo 
 [1] 2011-04-04 20:45:00 GMT 
  

嗯？我预计 GMT时间为2011-04-04 00:45:00

编辑：更令人困惑！如果我使用国家/城市而不是GMT偏移量指定时区，则结果会有所不同。

  foo<-as.POSIXct （ 2011-04-04 10:45:00，tz = Australia / Sydney）
 foo 
 [1] 2011-04-04 10:45:00 EST 
 attr（foo， tzone）<- GMT 
 foo 
 [1] 2011-04-04 00:45:00 GMT 
  

什么？啊！为什么？？

解决方案

格林尼治标准时间+10实际上表示从格林尼治标准时间开始使用POSIX样式符号减去 10小时。 / p>

来自： https：//www.ietf .org / timezones / data / etcetera

 ＃我们在区域名称和输出中使用POSIX样式的符号缩写
＃，尽管这与许多人的预期相反。 
＃POSIX在格林威治以西有积极的迹象，但许多人期望
＃在格林威治以东的积极迹象。例如，TZ ='Etc / GMT + 4'使用
＃的缩写 GMT + 4，并且比UT 
＃落后4小时（即格林威治以西），即使许多人希望这样做至
＃表示比UT提前4个小时（即格林威治以东）。 
  

I'm trying to understand how to interpret GMT offsets so I can work with datetime objects in R. For example, suppose I have a datetime like "2011-04-04 10:45:00 GMT+10"

Q1: Should I read that as a Grenwich time and add 10 hours to get the local time? Or is it a local time and I need to subtract 10 hours to get GMT? I always understood it was the latter.

Q2: Why does R seem to use the former interpretation? For example

foo <- as.POSIXct("2011-04-04 10:45:00", tz="Etc/GMT+10")
attr(foo, "tzone") <- "GMT"
foo
[1] "2011-04-04 20:45:00 GMT"

Huh? I expected "2011-04-04 00:45:00 GMT"

Edit: More confusing still! The result is different if I specify a timezone using Country/City rather than a GMT offset.

foo <- as.POSIXct("2011-04-04 10:45:00", tz="Australia/Sydney")
foo
[1] "2011-04-04 10:45:00 EST"
attr(foo, "tzone") <- "GMT"
foo
[1] "2011-04-04 00:45:00 GMT"

What? Argh! Why??

解决方案

GMT+10 actually means minus 10 hours from GMT using "POSIX Style Signs".

From: https://www.ietf.org/timezones/data/etcetera

# We use POSIX-style signs in the Zone names and the output abbreviations,
# even though this is the opposite of what many people expect.
# POSIX has positive signs west of Greenwich, but many people expect
# positive signs east of Greenwich.  For example, TZ='Etc/GMT+4' uses
# the abbreviation "GMT+4" and corresponds to 4 hours behind UT
# (i.e. west of Greenwich) even though many people would expect it to
# mean 4 hours ahead of UT (i.e. east of Greenwich).

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