[R] converting a time to nearest half-hour

[David Winsemius]

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