[R] Round down to earliest hour or half hour

[Schatzi]

I have times and would like to round down to the earliest 30 minute
increment. For instance, a time of 
2011-04-28 09:02:00
(the as.numeric value = 1303999320)
I would like it to be rounded down to:
2011-04-28 09:00:00
(the as.numeric value = 1303999200)

Any ideas of how to do this?

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In theory, practice and theory are the same. In practice, they are not - Albert Einstein
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