On Fri, 7 Nov 2008, Dennis Fisher wrote:
Colleagues,
I submitted this several days ago and no one responded, so I am trying
again, trying a different subject line:
Well, you posted something that indicated you had not studied the relevant
help pages, without the information requested in the R posting guide, and
with an HTML posting.
I just encountered some unexpected behavior of difftime in
relationship to the change from daylight savings to standard time.
My understanding is that DST and ST take effect at 2AM. However, the
results below suggests that R (version 2.8.0 in OS X) implements the
change at 2:16AM:
The transition time depends on the country (and in some cases, the year).
In the EU it is at 2am (and always has been, not that the EU is very old).
Expected:
difftime("2008-11-02 02:01:00", "2008-11-02 00:59:00")
Time difference of 2.033333 hours
difftime("2008-11-02 01:16:00", "2008-11-02 01:15:00")
Time difference of 1 mins
difftime("2008-11-02 01:18:00", "2008-11-02 01:17:00")
Time difference of 1 mins
Not expected:
difftime("2008-11-02 01:17:00", "2008-11-02 01:16:00")
Time difference of 1.016667 hours
Can anyone explain this?
Yes, and you should have been able to do so from the information in the
help pages. As ?strptime says
Remember that in most timezones some times do not occur and some
occur twice because of transitions to/from summer time. What
happens in those cases is OS-specific.
See also the note on ?Sys.timezone. No OS I tried did this, not even Mac
OS X set to EST5EDT. But in EST5EDT 2008-11-02 01:17:00 occurred twice:
as.POSIXct("2008-11-02 01:17:00")
[1] "2008-11-02 01:17:00 EDT"
as.POSIXct("2008-11-02 01:17:00") + 3600
[1] "2008-11-02 01:17:00 EST"
so 1.016667 hours would be one of two correct answers in that timezone.
However, you didn't tell us what timezone you are in and hence we cannot
know when DST transitions occur in that timezone.
Dennis
Dennis Fisher MD
P < (The "P Less Than" Company)
Phone: 1-866-PLessThan (1-866-753-7784)
Fax: 1-415-564-2220
www.PLessThan.com
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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