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copied!<p>This is a good question, and highlights some of the difficulty in dealing with dates in R. The lubridate package is very handy, so below I present two approaches, one using base (as suggested by @RJ-) and the other using lubridate. </p> <p>Recreate the (first two rows of) the dataframe in the original post:</p> <pre><code>foo <- data.frame(start.time = c("2012-02-06 15:47:00", "2012-02-06 15:02:00", "2012-02-22 10:08:00"), duration = c(1,2,3)) </code></pre> <p>Convert to POSIXct and POSIXt class (two ways to do this)</p> <pre><code># using base::strptime t.str <- strptime(foo$start.time, "%Y-%m-%d %H:%M:%S") # using lubridate::ymd_hms library(lubridate) t.lub <- ymd_hms(foo$start.time) </code></pre> <p>Now, extract time as decimal hours</p> <pre><code># using base::format h.str <- as.numeric(format(t.str, "%H")) + as.numeric(format(t.str, "%M"))/60 # using lubridate::hour and lubridate::minute h.lub <- hour(t.lub) + minute(t.lub)/60 </code></pre> <p>Demonstrate that these approaches are equal:</p> <pre><code>identical(h.str, h.lub) </code></pre> <p>Then choose one of above approaches to assign decimal hour to <code>foo$hr</code>:</p> <pre><code>foo$hr <- h.str # If you prefer, the choice can be made at random: foo$hr <- if(runif(1) > 0.5){ h.str } else { h.lub } </code></pre> <p>then plot using the ggplot2 package:</p> <pre><code>library(ggplot2) qplot(foo$hr, foo$duration) + scale_x_datetime(labels = "%S:00") </code></pre>

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