[R] POSIX revisited
Prof Brian Ripley
ripley at stats.ox.ac.uk
Wed Apr 25 14:39:36 CEST 2001
On Wed, 25 Apr 2001, Bernie McConnell wrote:
> 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
 "2000-01-12 12:00:00 GMT"
Obvious? It converts to POSIXct automatically, but that should be of no
> 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
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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