Storing time without date but not as class character - r

Question

I have a local.time column in my data frame of class character containing elements like this :
> a$local.time
[1] "1:30 AM" "6:29 AM" "6:59 AM" "9:54 AM" "10:14 AM" "10:34 AM" "12:54 PM" "1:15 PM" "1:20 PM"
[10] "1:20 PM" "2:15 PM" "2:15 PM" "4:23 AM" "6:28 AM" "2:45 PM" "3:08 PM" "3:23 PM" "3:58 PM"
I wanted to convert them from class character to time variables. So I used:
> as.POSIXct(a$local.time, tz = "", format = "%I:%M %p", usetz = FALSE)
This resulted in :
[1] "2014-10-31 01:30:00 EDT" "2014-10-31 06:29:00 EDT" "2014-10-31 06:59:00 EDT" "2014-10-31 09:54:00 EDT"
[5] "2014-10-31 10:14:00 EDT" "2014-10-31 10:34:00 EDT" "2014-10-31 12:54:00 EDT" "2014-10-31 13:15:00 EDT"
I have a date variable in a different column and the intention is to provide the capability of filtering by date and zooming on time bands to the minute in a dynamic dashboard.
I want to remove the date and time zone from a$local.time but keep it in a time format so the chronology is maintained i.e. 18:57 is the 19th hour and 57th minute of the day etc.
If I use
a$local.time <- format(a$local.time, "%Y-%m-%d %H:%M:%S", usetz = FALSE)
a$local.time <- strftime(a$local.time, format = "%H:%m") ,
the class changes to character ! What's the right approach?

Answer 1

The chron package has a "times" class that might be helpful for you. Starting with something similar to what you have so far:
x <- c("1:30 AM", "6:29 AM", "6:59 AM", "9:54 AM", "10:14 AM", "3:15 PM"))
a <- as.POSIXct(x, tz = "", format = "%I:%M %p", usetz = FALSE)
Then we can use the times function with format
library(chron)
(tms <- times(format(a, "%H:%M:%S")))
# [1] 01:30:00 06:29:00 06:59:00 09:54:00 10:14:00 15:15:00
attributes(tms)
# $format
# [1] "h:m:s"
#
# $class
# [1] "times"

Answer 2

You can use the hms (hour-minute-second) series of functions in the lubridate package.
library(lubridate)
times = c("1:30 AM", "6:29 AM", "6:59 AM", "9:54 AM", "2:45 PM")
I was hoping you could just do:
hm(times)
[1] "1H 30M 0S" "6H 29M 0S" "6H 59M 0S" "9H 54M 0S" "2H 45M 0S"
But notice that hm doesn't recognize the AM/PM distinction. So here's a more convoluted method that requires first using strptime, which does recognize AM/PM, and then putting the result in a form hm recognizes.
hm(paste0(hour(strptime(times, "%I:%M %p")),":",
minute(strptime(times, "%I:%M %p"))))
[1] "1H 30M 0S" "6H 29M 0S" "6H 59M 0S" "9H 54M 0S" "14H 45M 0S"
There's probably a better way, but this seems to work.
UPDATE: To address your comment, you can use the hour and minute functions to get the hours and minutes (although I like #RichardScriven's answer better). For example:
hour(times)
[1] 1 6 6 9 14
mean(hour(times) + minute(times)/60)
[1] 7.923333