converting a time to nearest half-hour

By entering "trunc.POSIXt" at the R commandline, you can see the standard truncate implementation. Riffing on this, 

roundhalfhour <- function( x ) {
 x <- as.POSIXlt( x + as.difftime( 15, units="mins" ) )
 x$sec <- 0
 x$min <- 30*(x$min %/% 30)
 as.POSIXct(x)
}

The as.double approach ought to work also, but rounding error might be a problem.

On Jul 23, 2010, at 11:35 AM, David Winsemius wrote:

On Jul 23, 2010, at 11:20 AM, <Murali.Menon at avivainvestors.com> <Murali.Menon at avivainvestors.com 

wrote:

Hi folks,

I've got a POSIXct datum as follows:

Sys.time()

[1] "2010-07-23 11:29:59 BST"

I want to convert this to the nearest half-hour, i.e., to  
"2010-07-23 11:30:00 BST"

(If the time were "11:59:ss", I want to convert to "12:00:00").

How to achieve this?

Couldn't you just coerce to numeric, divide by 60(sec)*30(half-hour  
minutes), round to integer, multiply by 60*30,  coerce to POSIXct?

When I tried my method I see that one also needs to add or subtract  
the proper number of seconds from Universal Time to get the output  
formatting correct. (Probably demonstrates that I do not have the  
proper understanding of the right place to employ a TZ specification.).

David Winsemius, MD
West Hartford, CT

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