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extract hours and seconds from POSIXct for plotting purposes in R

Suppose I have the following data.frame foo

           start.time duration
1 2012-02-06 15:47:00      1
2 2012-02-06 15:02:00      2
3 2012-02-22 10:08:00      3
4 2012-02-22 09:32:00      4
5 2012-03-21 13:47:00      5

And class(foo$start.time) returns

[1] "POSIXct" "POSIXt" 

I'd like to create a plot of foo$duration v. foo$start.time. In my scenario, I'm only interested in the time of day rather than the actual day of the year. How does one go about extracting the time of day as hours:seconds from POSIXct class of vector?