Is difftime a "class"

I am trying to write S4 methods with "difftime" in the signature but
am being "informed" (? not a warning or error) that "difftime" is not
a class. Nevertheless, dispatch takes place. Should I simply ignore
that "information"? Here is a toy example:

setClass("foo", contains = "Date")
setMethod("+", c("foo", "difftime"), function(e1, e2) callNextMethod())

in method for ?+? with signature ?"foo","difftime"?: no definition for
class ?difftime?
[1] "+"

x <- new("foo", as.Date("2013-06-30"))
dt <- as.difftime(1, units = "days")
x + dt

[1] "2013-07-01"

Thank you,
Dan Murphy