as.Date(as.POSIXct()) 给出错误的日期? [英] as.Date(as.POSIXct()) gives the wrong date?

问题描述

我一直在尝试查看一个数据框，提取 POSIXct 列的日期组件与某个值匹配的所有行.我遇到了以下内容，这让我非常困惑:: as.Date(as.POSIXct(...)) 并不总是返回正确的日期.

I'd been trying to look through a dataframe extracting all rows where the date component of a POSIXct column matched a certain value.I came across the following which is confusing me mightily:: as.Date(as.POSIXct(...)) doesn't always return the correct date.

> dt <- as.POSIXct('2012-08-06 09:35:23')
[1] "2012-08-06 09:35:23 EST"
> as.Date(dt)
[1] "2012-08-05"

为什么'2012-08-06 09:35:23'的日期等于'2012-08-05?

Why is the date of '2012-08-06 09:35:23' equal to '2012-08-05?

我怀疑这与所使用的不同时区有关，因此请注意 dt 的时区是EST"，我将其提供给 as.Date::

I suspect it's something to do with different timezones being used, so noting that the timezone of dt was 'EST' I gave this to as.Date::

> as.Date(as.POSIXct('2012-08-06 09:35:23'), tz='EST')
[1] "2012-08-05"

但它仍然返回 2012-08-05.

But it still returns 2012-08-05.

这是为什么?如何在我的数据框中找到日期为 2012-08-06 的所有日期时间?(as subset(my.df, as.character(as.Date(datetime), tz='EST') == '2012-08-06') 不返回日期时间 dt 即使这确实发生在日期 2012-08-06...)?

Why is this? How can I find all datetimes in my dataframe that were on the date 2012-08-06? (as subset(my.df, as.character(as.Date(datetime), tz='EST') == '2012-08-06') does not return the row with datetime dt even though this did occur on the date 2012-08-06...)?

添加的细节:Linux 64bit(虽然可以在 32bit 上重现)，可以在 R 3.0.1 和3.0.0，我目前是 AEST(澳大利亚东部标准时间)

Added details: Linux 64bit (though can reproduce on 32bit), can get this on both R 3.0.1 & 3.0.0, and I am currently AEST (Australian Eastern Standard Time)

推荐答案

安全的方法是通过 format 传递日期值.这确实创建了一个额外的步骤，但 as.Date 将接受使用-"或/"格式的字符结果:

The safe way to do this is to pass the date value through format. This does create an additional step but as.Date will accept the character result if it is formated with a "-" or "/":

as.Date( format( as.POSIXct('2019-03-11 23:59:59'), "%Y-%m-%d") )
[1] "2019-03-11"

as.Date(  as.POSIXct('2019-03-11 23:59:59') ) # I'm in a locale where the problem might exist
[1] "2019-03-12"

时区的文档也让我感到困惑.在某些情况下(事实证明在这种情况下)，EST 可能并不明确，实际上可能指的是澳大利亚的 tz.如果您恰好在北美，请尝试EST5EDT"或America/New_York".

The documentation for timezones is confusing to me too. In some (and this case as it turned out) case EST may not be unambiguous and may actually refer to a tz in Australia. Try "EST5EDT" or "America/New_York" if you happen to be in North America.

在这种情况下，它还可能与您未说明的操作系统处理tz"参数的方式不同有关，因为我得到2012-08-06".(我目前在 PDT US tz 中，虽然我不确定这是否重要.)更改获取 tz 参数的函数可能会澄清(或不澄清):

In this case it could also relate to differences in how your unstated OS handles the 'tz' argument, since I get "2012-08-06". ( I'm in PDT US tz at the moment, although I'm not sure that should matter. )Changing which function gets the tz argument may clarify (or not):

> as.Date(as.POSIXct('2012-08-06 19:35:23', tz='EST'))
[1] "2012-08-07"
> as.Date(as.POSIXct('2012-08-06 17:35:23', tz='EST'))
[1] "2012-08-06"


> as.Date(as.POSIXct('2012-08-06 21:35:23'), tz='EST')
[1] "2012-08-06"
> as.Date(as.POSIXct('2012-08-06 22:35:23'), tz='EST')
[1] "2012-08-07"

如果您从 as.POSIXct 中省略 tz，则假定为 UTC.

If you omit the tz from as.POSIXct then UTC is assumed.

这些是 Ozzie TZ 的明确名称(至少在我的 Mac 上):

These are the unambiguous names of the Ozzie TZ's (at least on my Mac):

tzfile <- "/usr/share/zoneinfo/zone.tab"
tzones <- read.delim(tzfile, row.names = NULL, header = FALSE,
    col.names = c("country", "coords", "name", "comments"),
    as.is = TRUE, fill = TRUE, comment.char = "#")
grep("^Aus", tzones$name, value=TRUE)
 [1] "Australia/Lord_Howe"   "Australia/Hobart"     
 [3] "Australia/Currie"      "Australia/Melbourne"  
 [5] "Australia/Sydney"      "Australia/Broken_Hill"
 [7] "Australia/Brisbane"    "Australia/Lindeman"   
 [9] "Australia/Adelaide"    "Australia/Darwin"     
[11] "Australia/Perth"       "Australia/Eucla" 

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