将 POSIXct 日期时间转换为日期时出现意外日期 - 时区问题? [英] Unexpected date when converting POSIXct date-time to Date - timezone issue?

问题描述

当我尝试使用 as.Date 将 POSIXct 日期时间强制为 Date 时，它似乎返回了错误的日期.

When I try to coerce a POSIXct date-time to a Date using as.Date, it seems to return wrong date.

我怀疑这与时区有关.我尝试了 as.Date 中的 tz 参数，但它没有给出预期的日期.

I suspect it has got something to do with the time zone. I tried the tz argument in as.Date, but it didn't give the expected date.

# POSIXct returns day of month 24  
data$Time[3]
# [1] "2020-03-24 00:02:00 IST"

class(data$Time[3])
# [1] "POSIXct" "POSIXt"

# coerce to Date, returns 23 
as.Date(data$Time[3])
# [1] "2020-03-23"

# try the time zone argument, without luck
as.Date(data$Time[3], tz = "IST")
# [1] "2020-03-23"
# Warning message:
# In as.POSIXlt.POSIXct(x, tz = tz) : unknown timezone 'IST' 

Sys.timezone()
# [1] "Asia/Calcutta"

您有什么想法吗?

推荐答案

使用最后注释中的设置，我们可以使用以下任何一种:

Using the setup in the Note at the end we can use any of these:

# same date as print(x) shows
as.Date(as.character(x))
## [1] "2020-03-24"

# use the time zone stored in x (or system time zone if that is "")
as.Date(x, tz = attr(x, "tzone"))
## [1] "2020-03-24"

# use system time zone
as.Date(x, tz = "")
## [1] "2020-03-24"

# use system time zone
as.Date(x, tz = Sys.timezone())
## [1] "2020-03-24"

# use indicated time zone
as.Date(x, tz = "Asia/Calcutta")
## [1] "2020-03-24"

注意

我们假设了这个设置.

Note

We have assumed this setup.

Sys.setenv(TZ = "Asia/Calcutta")
x <- structure(1584988320, class = c("POSIXct", "POSIXt"), tzone = "")

R.version.string
## [1] "R version 4.0.2 Patched (2020-06-24 r78745)"

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