[R] how to work with time of day (independent of date)

[Daniel Nordlund]

I have a data frame with date/times represented as charaacter strings 
and and a value at that date/time.  I want to get the mean value for 
each time of day, across days, and then plot time of day on the x-axis 
and means on the y-axis.  R doesn't appear to have a built-in time of 
day time type (independent of a date), unless I have missed something. 
What is the best way to create a time variable so that I can aggregate 
and plot by time of day, with time labelled in HH:MM format.  My current 
approach is to convert all date/times to the same date.  I can then 
manage the rest of what I want with ggplot2.  But I am  wondering if 
there is an easier/better way to do deal with time of day.

Here is a sample data frame.

df <- structure(list(date = structure(1:8, .Label = c("2015-10-29 
00:50:00",
"2015-10-29 09:30:00", "2015-10-29 21:10:00", "2015-10-30 00:50:00",
"2015-10-30 09:30:00", "2015-10-30 21:10:00", "2015-10-31 00:50:00",
"2015-10-31 10:30:00"), class = "factor"), value = c(88L, 17L,
80L, 28L, 23L, 39L, 82L, 79L)), .Names = c("date", "value"), row.names = 
c(NA,
-8L), class = "data.frame")


Any suggestions appreciated.

Dan

-- 
Daniel Nordlund
Bothell, WA  USA