dates <- as.POSIXct(as.Date("2011/01/01") + sample(0:365,100, replace=TRUE))

times <- as.POSIXct(runif(100, 0, 24*60*60),origin="2011/01/01")

df <- data.frame(dates = dates,times = times)

library(ggplot2)

library(scales)

ggplot(df, aes(x=dates, y=times)) +

   geom_point() +

  scale_y_datetime(breaks=date_breaks("4 hour"),labels=date_format("%H:%M")) +