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rtimezoneposix

How to extract the correct timezones from POSIXct and POSIXlt objects?

time1 = as.POSIXlt("2010-07-01 16:00:00", tz="Europe/London")
time1
# [1] "2010-07-01 16:00:00 Europe/London"

but

time2 = as.POSIXct("2010-07-01 16:00:00", tz="Europe/London")
time2
# [1] "2010-07-01 16:00:00 BST"

Why is the timezone presented differently? It is important for me because I need to extract the time zones from my date.

base::format(time1, format="%Z")
# [1] "BST"
base::format(time2, format="%Z")
# [1] "BST"

both give the same "BST" for British Saving Time!

The issue is that "BST" does not seam to be recognized by POSIXct/POSIXlt format:

as.POSIXlt("2010-07-01 16:00:00", tz="BST")
# [1] "2010-07-01 16:00:00 BST"
# Warning messages:
# 1: In strptime(xx, f <- "%Y-%m-%d %H:%M:%OS", tz = tz) :
#   unknown timezone 'BST'
# 2: In structure(xx, class = c("POSIXct", "POSIXt"), tzone = tz) :
#   unknown timezone 'BST'
# 3: In strptime(x, f, tz = tz) : unknown timezone 'BST'
as.POSIXct("2010-07-01 16:00:00", tz="BST")
# [1] "2010-07-01 16:00:00 GMT"
# Warning messages:
# 1: In strptime(xx, f <- "%Y-%m-%d %H:%M:%OS", tz = tz) :
#   unknown timezone 'BST'
# 2: In structure(xx, class = c("POSIXct", "POSIXt"), tzone = tz) :
#   unknown timezone 'BST'
# 3: In strptime(x, f, tz = tz) : unknown timezone 'BST'
# 4: In structure(xx, class = c("POSIXct", "POSIXt"), tzone = tz) :
#   unknown timezone 'BST'
# 5: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'BST'

I am really confused. I have 2 questions:

1/ What is the difference between POSIXct and POSIXlt formats

2/ Any one knows what time zone I can use?

"Europe/London" works with POSIXlt but not POSIXct. Plus it cannot be extracted from a time using base::format
"BST" is not recognized as a valid timezone in as.POSIXct or as.POSIXlt functions.

Solution

  • @Koshke showed you already

    • the difference in internal representation of both date types, and
    • that internally, both timezone specifications are the same.

    You can get the timezone out in a standardized manner using attr(). This will get the timezone in the form specified in the zone.tab file, which is used by R to define the timezones (More info in ?timezones ).

    eg : 

    > attr(time1,"tzone")
[1] "Europe/London"
> attr(time2,"tzone")
[1] "Europe/London"

    I am quite amazed though that POSIXct uses different indications for the timezones than POSIXlt, whereas the attributes are equal. Apparently, this "BST" only pops up when the POSIXct is printed. Before it gets printed, POSIXct gets converted again to POSIXlt, and the tzone attribute gets amended with synonyms :

    > attr(as.POSIXlt(time2),"tzone")
[1] "Europe/london" "GMT"           "BST"

    This happens somewhere downstream of the internal R function as.POSIXlt, which I'm not able to look at for the moment due to more acute problems to solve. But feel free to go through it and see what exactly is going on there.

    On a sidenote, "BST" is not recognized as a timezone (and it is not mentioned in zone.tab either) on my Windows 7 / R 2.13.0 install.