问题
When I try to coerce a POSIXct
date-time to a Date
using as.Date
, it seems to return wrong date.
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"
Any ideas what may be going wrong here?
回答1:
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"
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)"
回答2:
The clue is in the warning message. as.Date()
doesn't know how to interpret IST
as a timezone and so defaults to UTC
. Assuming that IST
is Indian Standard Time (rather than Irish Standard time) and that IST is UTC+5:30, as.Date()
is giving the expected result, even if it is incorrect for your purposes.
Providing a date with a timezone expressed as an offset from UTC gives the desired result.
as.Date("2020-03-24 00:02:00 UTC+5:30")
[1] "2020-03-24"
来源:https://stackoverflow.com/questions/62743019/unexpected-date-when-converting-posixct-date-time-to-date-can-timezone-fix-it