If you want to convert all elements of
a to a single numeric vector and
length(a) is greater than 1 (OK, even if it is of length 1), you could
unlist the object first and then convert.
as.numeric(unlist(a)) #  10 38 66 101 129 185 283 374
Bear in mind that there aren’t any quality controls here. Also,
X$Days a mighty odd name.