In the R-Lang practice exercises, they have this:

some_time <- as.POSIXct("2016-0801 15:00:00")
unclass(some_time)

They then ask about the type of object it returns. It turns out that it returns a Unix-style timestamp. I don't recall any in-course content that covered that or in-course exercises that led toward exploring that.

It's okay. I'm running RStudio on another laptop, so I just entered the code and looked at the result (1470063600, with a "tzone" attribute).