[R] Timezone issue with strftime/strptime and %z and %Z

[Daniel Nüst]

2011/3/11 David Winsemius <dwinsemius at comcast.net>:
> On Mar 10, 2011, at 8:46 PM, David Winsemius wrote:
>> On Mar 10, 2011, at 11:17 AM, Daniel Nüst wrote:
>>> I try to parse a time stamp with time zone. I essentially just want to
>>> parse the time stamp "1995-05-25T15:30:00+10:00" and output it exactly
>>> like it is, using the POSIX classes (or is that impossible?).
>>
>> Does this work?
>>
>> as.POSIXlt(gsub("T.*(\\+|\\-)..(:)", "", # get rid of the colon in the tz
>>          # but preserve the sign for the %z format string
>>          gsub("T", " ", "1995-05-25T15:30:00-1000")), # replace the "T"
>> with a space
>>           format="%Y-%m-%d %H:%M:%S%z")
>
> Daniel;
>
> That example didn't have the full complexity of the question so I didn't
> notoce that I had incorrectly constructed a regex OR within the colon
> handling clause. I neverdid figure out how to do that properly, but htis
> should handle it:
>
> as.POSIXlt(gsub("T", " ", #change T to space
> +           # but preserve the sign for the %z format string
> +           gsub("(T..:..:.....):", "\\1", "1995-05-25T15:30:00-10:00")),
>  format="%Y-%m-%d %H:%M:%S%z")
> [1] "1995-05-25 21:30:00"
>
> To get output in GMT add tz argument to as.POSIXlt:
>
>> as.POSIXlt(gsub("T", " ", #change T to space
> +           # but preserve the sign for the %z format string
> +           gsub("(T..:..:.....):", "\\1", "1995-05-25T15:30:00-10:00")),
>  format="%Y-%m-%d %H:%M:%S%z", tz="GMT")
> [1] "1995-05-26 01:30:00 GMT"

This procudes the same output on my machine, but still does not solve
the problem, which lies on the output side, sorry if I was not clear
about that before:

time <- as.POSIXlt(gsub("T", " ", 	# change T to space
				# but preserve the sign for the %z format string
				gsub("(T..:..:.....):", "\\1", "1995-05-25T15:30:00-10:00")),
		format="%Y-%m-%d %H:%M:%S%z") #, tz="GMT")
format(x = time, format = "%Y-%m-%d %H:%M:%S%z")
# [1] "1995-05-26 01:30:00Mitteleuropäische Zeit"
strftime(x = time, format = "%Y-%m-%d %H:%M:%S%z")
# [1] "1995-05-26 03:30:00Mitteleuropäische Sommerzeit"

But I need "...01:30:00+01:00", or "...01:30:00+0100", as I expect
from %z because in ?strptime I read:

"%z
    Signed offset in hours and minutes from UTC, so -0800 is 8 hours
behind UTC."

and

"Note that when %z or %Z is used for output with an object with an
assigned timezone an attempt is made to use the values for that
timezone — but it is not guaranteed to succeed."

Does this just mean it just won't work?

Let me rephrase my question: How can I create a time object from the
character string "1995-05-25T15:30:00-10:00" and get exactly the same
character string again when formatting it/printing it?

Thanks for any help!
/Daniel

>>>
>>>> sessionInfo()
>>>
>>> R version 2.12.1 (2010-12-16)
>>> Platform: x86_64-pc-mingw32/x64 (64-bit)
>>>
>>> locale:
>>> [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252
>>> [3] LC_MONETARY=German_Germany.1252 LC_NUMERIC=C
>>> [5] LC_TIME=German_Germany.1252
>>>
>>> attached base packages:
>>> [1] stats     graphics  grDevices utils     datasets  methods   base
>>>
>>> other attached packages:
>>> [1] rj_0.5.2-1
>>>
>>> loaded via a namespace (and not attached):
>>> [1] rJava_0.8-8  tools_2.12.1
>>>>
>>>> t1 <- strptime("1995-05-25T15:30:00+10:00", format =
>>>> "%Y-%m-%dT%H:%M:%OS")
>>>> t2 <- strptime("1995-05-25T15:30:00+10:00", format =
>>>> "%Y-%m-%dT%H:%M:%OS%z")
>>>>
>>>> strftime(t1, format = "%Y-%m-%dT%H:%M:%OS")
>>>
>>> [1] "1995-05-25T15:30:00"
>>>>
>>>> strftime(t1, format = "%Y-%m-%dT%H:%M:%OS%z")
>>>
>>> [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> # Ends in "Mitteleuropäische Sommerzeit", not in +10:00, so time zone is
>>>> ignored!
>>>> # Also no difference beetween %z and %z !
>>>> strftime(t1, format = "%Y-%m-%dT%H:%M:%OS%Z")
>>>
>>> [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> # All this does NOT remove the "Mitteleuropäische Zeit" from the
>>>> strftime output!!
>>>>
>>>> # Can locale solve the problem?
>>>> Sys.getlocale(category = "LC_TIME")
>>>
>>> [1] "German_Germany.1252"
>>>>
>>>> Sys.setlocale("LC_TIME", "English")
>>>
>>> [1] "English_United States.1252"
>>>>
>>>> strftime(t1, format = "%Y-%m-%dT%H:%M:%OS%z")
>>>
>>> [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> # [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit" -- No change.
>>>>
>>>> # does t1 actually have time zone?
>>>> attributes(t1)
>>>
>>> $names
>>> [1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"
>>>  "isdst"
>>>
>>> $class
>>> [1] "POSIXlt" "POSIXt"
>>>
>>>>
>>>> format(t1, format = "%Y-%m-%dT%H:%M:%OS%z") # usetz = TRUE) # no change
>>>> on usetz
>>>
>>> [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> # Is the : in offset the problem?
>>>> t3 <- strptime("1995-05-25T15:30:00+1000", format =
>>>> "%Y-%m-%dT%H:%M:%S%z")
>>>> attributes(t3)
>>>
>>> $names
>>> [1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"
>>>  "isdst"
>>>
>>> $class
>>> [1] "POSIXlt" "POSIXt"
>>>
>>>> format(t3, format = "%Y-%m-%dT%H:%M:%OS%z")
>>>
>>> [1] "1995-05-25T07:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> # [1] "1995-05-25T07:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> strftime(t1, format = "%Y-%m-%dT%H:%M:%OS%z", tz = "+0200") # no effect
>>>> on setting tz
>>>
>>> [1] "1995-05-25T15:30:00Mitteleuropäische Sommerzeit"
>>>>
>>>> Sys.setenv(TZ="GMT") # no working effect on format and strftime
>>>
>> --
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>> ______________________________________________
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>> PLEASE do read the posting guide
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>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> West Hartford, CT
>
>