Incorrect formatted output after subtracting non-integer seconds from POSIXt origin

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x<-as.POSIXct("1970-1-1", tz="UTC")-.5
y<-as.POSIXct("1970-1-1", tz="UTC")+.5
x==y
[1] FALSE  # of course
but x and y "appear" to be the same when formatted, even with extra
precision:
format(x, format="%Y-%m-%d %H:%M:%OS2")
[1] "1970-01-01 00:00:00.50"
format(y, format="%Y-%m-%d %H:%M:%OS2")
[1] "1970-01-01 00:00:00.50"

Formatted output is fine for integral difference points ...
x<-as.POSIXct("1970-1-1", tz="UTC")-1
y<-as.POSIXct("1970-1-1", tz="UTC")+1
format(x, format="%Y-%m-%d %H:%M:%OS2")
[1] "1969-12-31 23:59:59.00"
format(y, format="%Y-%m-%d %H:%M:%OS2")
[1] "1970-01-01 00:00:01.00"

... but seems to be a second "off" for non-integers:
format(as.POSIXct("1970-1-1", tz="UTC")-1.5, format="%Y-%m-%d %H:%M:%OS2")
[1] "1969-12-31 23:59:59.50"   # a second later than expected
format(as.POSIXct("1970-1-1", tz="UTC")-86400, format="%Y-%m-%d
%H:%M:%OS2")
[1] "1969-12-31 00:00:00.00"   # OK
format(as.POSIXct("1970-1-1", tz="UTC")-86400.5, format="%Y-%m-%d
%H:%M:%OS2")
[1] "1969-12-31 00:00:00.50"   # why "after" previous time?

Bug, or user misunderstanding?
If negative POSIX time is supposed to work then it's a bug in as.POSIXlt(). Now fixed in R-devel (and patched).

Cheers,
Simon

before:
str(unclass(as.POSIXlt(as.POSIXct("1970-1-1", tz="UTC")-0.2)))
List of 9
 $ sec  : num 0.8
 $ min  : int 0
 $ hour : int 0
 $ mday : int 1
 $ mon  : int 0
 $ year : int 70
 $ wday : int 4
 $ yday : int 0
 $ isdst: int 0
 - attr(*, "tzone")= chr "UTC"

R-devel:
str(unclass(as.POSIXlt(as.POSIXct("1970-1-1", tz="UTC")-0.2)))
List of 9
 $ sec  : num 59.8
 $ min  : int 59
 $ hour : int 23
 $ mday : int 31
 $ mon  : int 11
 $ year : int 69
 $ wday : int 3
 $ yday : int 364
 $ isdst: int 0
 - attr(*, "tzone")= chr "UTC"
"R version 2.11.1 (2010-05-31)" on Windows
Thanks.
Dan Murphy

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