R dates "origin" must be supplied

I suspect you meant:

axis.Date(1, as.Date(sites$date, origin = "1970-01-01"))

as the 'x' argument to as.Date() has to be of type Date.

As an aside, this would have been appropriate as a follow-up or edit of your previous question.

Another option is the lubridate package:

library(lubridate)

x <- 15103
as_date(x, origin = lubridate::origin)
"2011-05-09"

y <- 1442866615
as_datetime(y, origin = lubridate::origin)
"2015-09-21 20:16:55 UTC"

From the docs:

Origin is the date-time for 1970-01-01 UTC in POSIXct format. This date-time is the origin for the numbering system used by POSIXct, POSIXlt, chron, and Date classes.

My R use 1970-01-01:

>as.Date(15103, origin="1970-01-01")
[1] "2011-05-09"

and this matches the calculation from

>as.numeric(as.Date(15103, origin="1970-01-01"))