Hi, and welcome!
The first step will be importing your data into an R data frame. Then confirm that your PM column is a date class
library(lubridate)
#>
#> Attaching package: 'lubridate'
#> The following object is masked from 'package:base':
#>
#> date
leave <- ymd_hms("2011-08-10 14:00:00", tz = "Pacific/Auckland")
class(leave)
#> [1] "POSIXct" "POSIXt"
Created on 2019-12-19 by the reprex package (v0.3.0)
You'll want to install lubridate. The package has since been updated, but the introduction will give you a good orientation.
Once you have PM right, you'll be working with duration objects.
When you get stuck, come back with a new post including a reproducible example, called a reprex for more help.
Good decision to learn R! (I'm biased, as a long-term user.) I hope you find it a useful and fun as I have.