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

David Winsemius dwinsemius at comcast.net
Fri Mar 11 15:15:08 CET 2011 

On Mar 11, 2011, at 8:54 AM, Daniel Nüst wrote:

> 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?

 > x <- 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")
 > x
[1] "1995-05-26 01:30:00 GMT"
 > format(x, "%Y-%m-%d %H:%M:%S%z")
[1] "1995-05-26 01:30:00+0000"
 > format(x, "%Y-%m-%dT%H:%M:%S%z")
[1] "1995-05-26T01:30:00+0000"

It is easy todeal with the "T" separator. You did say that the ":" was  
optional in the output, didn't you?

-- 
David.
>
> 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
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT

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