The "epoch" is named in the documentation for as.POSIX, but it's definitely buried in there. I only found it because I knew it beforehand and used the find feature to search. Note that the epoch is defined just as an example, like it's not a useful thing to know.
Some of the concepts used have to be extended backwards in time (the usage is said to be ‘proleptic’). For example, the origin of time for the "POSIXct" class, ‘1970-01-01 00:00.00 UTC’, is before UTC was defined.
Anyways, you can always find the origin by taking a date (stored as number of days since the origin) and subtracting it's numeric form from itself. This will take you that many days backwards, leaving you at 0 days since the origin.
as.Date("2016-12-06") - as.numeric(as.Date("2016-12-06"))
# [1] "1970-01-01"