Extracting time from POSIXct

Question

How would I extract the time from a series of POSIXct objects discarding the date part?

For instance, I have:

times <- structure(c(1331086009.5009         


        

Answer 1

There have been previous answers that showed the trick. In essence:

• you must retain POSIXct types to take advantage of all the existing plotting functions

• if you want to 'overlay' several days worth on a single plot, highlighting the intra-daily variation, the best trick is too ...

• impose the same day (and month and even year if need be, which is not the case here)

which you can do by overriding the day-of-month and month components when in POSIXlt representation, or just by offsetting the 'delta' relative to 0:00:00 between the different days.

So with times and val as helpfully provided by you:

## impose month and day based on first obs
ntimes <- as.POSIXlt(times)    # convert to 'POSIX list type'
ntimes$mday <- ntimes[1]$mday  # and $mon if it differs too
ntimes <- as.POSIXct(ntimes)   # convert back

par(mfrow=c(2,1))
plot(times,val)   # old times
plot(ntimes,val)  # new times

yields this contrasting the original and modified time scales: