[R] c() and dates

[Therneau, Terry M., Ph.D.]

I'm a bit puzzled by a certain behavior with dates.  (R version 3.1.1)

 > temp1 <- as.Date(1:2, origin="2000/5/3")
 > temp1
[1] "2000-05-04" "2000-05-05"

 > temp2 <- as.POSIXct(temp1)
 > temp2
[1] "2000-05-03 19:00:00 CDT" "2000-05-04 19:00:00 CDT"

So far so good.  On 5/4, midnight in Greenwich it was 19:00 on 5/3 in my time zone.  The 
manual page has a clear explanation of what goes on.

 > c(temp1, temp2)
[1] "2000-05-04"    "2000-05-05"    "2623237-10-15" "2623474-05-06"
 > class(c(temp1, temp2))
[1] "Date"

 > c(temp2, temp1)
[1] "2000-05-03 19:00:00 CDT" "2000-05-04 19:00:00 CDT"
[3] "1969-12-31 21:04:41 CST" "1969-12-31 21:04:42 CST"
 > class(c(temp2, temp1))
[1] "POSIXct" "POSIXt"

I would have expected c() to determine a common class, somehow, then do the conversion and 
concatonate.  That is obviously not what happens.  I've read the manual page but I must be 
missing something.  I make no claim that R is broken, mistaken, or otherwise deficient, 
only that my understanding is so.

Could someone illuminate?

Terry T.