[Rcpp-devel] Setting time zone on Rcpp::Datetime

[Dirk Eddelbuettel]

On 12 January 2015 at 13:10, janus Larsen wrote:
| Hi again,
| 
| Thanks Dirk for your reply. So there is no way of doing this on the C side?
| 
| The thing is that I'm returning a large object that contains a lot of
| information (about an unstructured 3D model) and some of the fields are
| DateTimes. I could of cource wrap the call to the C function in an R function
| that then corrects the dates to GMT (or sets the timezone before calling the C
| function), but it just not that nice... Previously I had this code written
| using <RDefines.h> where I could set the timezone on the returned posIXct
| object:
| 
| SEXP long2DateTime(long dt) {
| SEXP result;
| result = PROTECT(allocVector(REALSXP, 1));
| REAL(result)[0] = dt;
| SEXP gmt = PROTECT(allocVector(STRSXP,1));
| SET_STRING_ELT(gmt, 0, mkChar("GMT"));
| SEXP tzone = PROTECT(allocVector(STRSXP,1));
| SET_STRING_ELT(tzone, 0, mkChar("tzone"));
| setAttrib(result, tzone, gmt);
| SEXP datetimeclass = PROTECT(allocVector(STRSXP,2));
| SET_STRING_ELT(datetimeclass, 0, mkChar("POSIXt"));
| SET_STRING_ELT(datetimeclass, 1, mkChar("POSIXct"));
| setAttrib(result, R_ClassSymbol, datetimeclass);
| UNPROTECT(4);
| return result;
| }

Oh dear. See eg http://gallery.rcpp.org/articles/creating-xts-from-c++/ or
the other few xts related answers on the Rcpp Gallery.

Dirk
 
| Janus
| 
| On Fri, Jan 9, 2015 at 1:22 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
| 
|    
|     On 9 January 2015 at 11:50, janus Larsen wrote:
|     | Hi,
|     | How do I set the timezone on an Rcpp::Datetime?
|     | Thanks in advance,
|     | Sunaj
|     |
|     | This returns "1970-01-01 01:00:00 CET" (computer setting), but I want
|     GMT...
| 
|     R does the formatting in its session based on your locale.
| 
|     That is generally the right thing:
|    
|     | // [[Rcpp::export]]
|     | Rcpp::Datetime test() {
|     |   double d=0;
|     |   return(d);
|     | }
| 
|     Small corrections to your code to actually export the function (under a
|     safer
|     name):
| 
|     #include <Rcpp.h>
| 
|     // [[Rcpp::export]]
|     Rcpp::Datetime timetest(double d=0) {
|       return(d);
|     }
| 
|     Then:
| 
|     R> sourceCpp("/tmp/timeQ.cpp")
|     R> timetest()                       # my default is Chicago, or -5
|     [1] "1969-12-31 18:00:00 CST"
|     R> as.numeric(timetest())
|     [1] 0
|     R> Sys.setenv("TZ"="Europe/London")
|     R> timetest()                       # I can select another one
|     [1] "1970-01-01 01:00:00 BST"
|     R> Sys.setenv("TZ"="UTC")
|     R> timetest()                       # incl UTC
|     [1] "1970-01-01 UTC"
|     R>
|     R> format(timetest(), tz="America/Chicago")
|     [1] "1969-12-31 18:00:00"
|     R>
| 
|     So you need to change the timezone _at the level of your app_ which can be
|     as
|     simple as writing a new Date formatter in R as per my last line.
|    
|     Dirk
| 
|     --
|     http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
| 
| 

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org