The arithmetic that David describes should work fine (POSIXct is
internally in UTC) unless you are in the Chatham Islands (which has a
UTC+12:45 time zone [1]) or Nepal (UTC+05:45 [2]) or some such place
with a funny idea about when the 1/2 hour mark is.
The formatting of the output may be tricky, if the original poster
really want the (past/future) time zone as opposed to the current one,
which is what his text could be read to suggest.
Good test cases would be "2010-03-28 01:46:00 GMT" which should
(presumably) round to "2010-03-28 03:00:00 BST" and "2010-10-31 02:46
BST" which probably rounds to "2010-10-31 02:00 GMT". I can't quite get
it to work in R, but it should be possible....
Allan
[1] https://secure.wikimedia.org/wikipedia/en/wiki/UTC%2B12:45
[2] https://secure.wikimedia.org/wikipedia/en/wiki/UTC%2B05:45
On 23/07/10 16:35, David Winsemius wrote:
On Jul 23, 2010, at 11:20 AM, <murali.me...@avivainvestors.com>
<murali.me...@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?
_
David Winsemius, MD
West Hartford, CT
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