R help - Apr 2001 - POSIX revisited

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On Wed, 25 Apr 2001, Bernie McConnell wrote:
> G'Day,
>
>
> I am still a little confused about manipulating date-time classes.  In the
example below I add 20 days to a POSIXlt object (x.lt) by adding 20 to
x.lt$mday.  But, not surprisingly, the mon and year components are not updated
accordingly.  Thus I convert to POSIXct and back again to POSIXlt and all is
fine.  Is this the recommended way of incrementing POSIXlt objects, or have I
missed something fundamental or obvious?

You can just add:
> x.lt + 20*86400
[1] "2000-01-12 12:00:00 GMT"

Obvious?  It converts to POSIXct automatically, but that should be of no
importance.

> A follow-on and more vague question is - I have found that sometimes
POSIXct
insists on displaying my cherished GMT-based  data collected in the summer as
"GMT Daylight Time" rather than Standard Time. Change Dec to Jun in
the example
to see this. How can I force POSIXct to GMT Standard Time - whatever the season.
 Seals don't use "Daylight Time" :-)

If you want the times in GMT and not in the local time zone, you have to ask for
it.  Just use

format(x, tx="GMT")

The use of "GMT Daylight Time" not BST is a Windows anachronism.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Summary of POSIX date-time thoughts ...
>> Brian,
>>
>> May I continue the POSIX thread a little more ...
>>
>> Is there a natural or correct way to represent just time of day, with
no
>> associated date, in the POSIX scheme?  Is setting date to 1970-01-01
the
>> best / only way?
>
>The best.
>
>> You suggest format(x, tx="GMT") to force POSIX objects to
appear as GMT.  I
>> take it that this disables the implementation of 'Daylight
Time'?  I am
>> unsure whether  'GMT Standard Time' and 'GMT Daylight
Time' are different
>> time zones or are they both just part of 'GMT' time zone?
>
>GMT means GMT. There is no such thing as `GMT Daylight Time': Windows is
>wrong here.  There is GMT/UTC and there is BST.
>
>> Would it be useful to be able e to be globally set all POSIX objects to
>> display as, say, 'GMT Standard ' with a 'global' in
options() ?
>
>You just set the timezone in use to that.  Sys.putenv(TZ="GMT")
for me.
>I'll look into seeing if there is an easier way.
>
>> Finally, I think the that the handling of date-times by POSIX object
>> warrants several paragraphs in 'An Introduction to R' - or
perhaps I'm just
>> slow on the uptake!
>
>It's too specialized.  There will be an article in the next R-news,
though.
...
...
>
>-- 
>Brian D. Ripley,                  ripley at stats.ox.ac.uk
>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 272860 (secr)
>Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>
>
>
Bernie McConnell
NERC SEA MAMMAL RESEARCH UNIT
Gatty Marine Lab, St Andrews University
St Andrews, Fife,  KY16 8LB, Scotland.

b.mcconnell at smru.st-andrews.ac.uk
http://www.smru.st-and.ac.uk
tel:  	+44 (1334) 463280   
fax:  	+44 (1334) 462632   


                
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