[R] converting a time to nearest half-hour

[Murali.Menon at avivainvestors.com]

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? 

Thanks,

Murali