See ?DateTimeClasses, ?strptime, and ?as.character
This example from strptime should get you going:
## read in date/time info in format 'm/d/y h:m:s'
dates <- c("02/27/92", "02/27/92", "01/14/92",
"02/28/92", "02/01/92")
times <- c("23:03:20", "22:29:56", "01:03:30",
"18:21:03", "16:56:26")
x <- paste(dates, times)
z <- strptime(x, "%m/%d/%y %H:%M:%S")
z
Good luck, Andy