为什么转换时区不能与as.POSIXct…一起使用？[R]

Question

I've read this and it says as.POSIXct is always UTC internally. No wonder I got

> time1 = as.POSIXct('2015-10-25 10:15:13 UTC')
> time1
[1] "2015-10-25 10:15:13 EDT"
# missing tz causes coercion (not converting!) to computer's tz. 
# (I'm in EDT Boston and calculating some data in Dubai time)


> time1 = as.POSIXct('2015-10-25 10:15:13 UTC', tz = 'UTC', usetz = T)
> time1
[1] "2015-10-25 10:15:13 UTC"

# not work
> as.POSIXct(time1, tz = 'Asia/Dubai', usetz = T)
[1] "2015-10-25 10:15:13 UTC"

# works but the result is character
> format(time1, tz = 'Asia/Dubai', usetz = T)
[1] "2015-10-25 14:15:13 GST"
> class(format(time1, tz = 'Asia/Dubai', usetz = T))
[1] "character"

I can use format , but it yields character and I can't use it in plotting something vs time. How can I plot with local time?

Answer 1

tried something and this works:

> library(lubridate)
> time1 = as.POSIXct('2015-10-25 10:15:13 UTC', tz = 'UTC', usetz = T)
> time1
[1] "2015-10-25 10:15:13 UTC"
> force_tz(time1 + 3600*4, tz = 'Asia/Dubai')
[1] "2015-10-25 14:15:13 GST"
> class(force_tz(time1 + 3600*4, tz = 'Asia/Dubai'))
[1] "POSIXct" "POSIXt" 

Also doable with Rbase with a lightly longer syntax (thanks @David Arenburg)

> time1 = as.POSIXct('2015-10-25 10:15:13 UTC', tz = 'UTC', usetz = T)
> time1
[1] "2015-10-25 10:15:13 UTC"
> as.POSIXct(format(time1, tz = 'Asia/Dubai', usetz = T), tz = 'Asia/Dubai', usetz = T)
[1] "2015-10-25 14:15:13 GST"