[R] r help date format changes with c() vs. rbind()

[Tim Clark]

Dear List,

I am having a problem with dates and I would like to understand what is going on.  Below is an example.  I can produce a date/time using as.POSIXct, but I am trying to combine two as.POSIXct objects and keep getting strange results.  I thought I was using the wrong origin, but according to structure(0,class="Date") I am not (see below).  In my example a is a simple date/time object, b combines it using rbind(), c converts b to a date/time object again using as.POSIXct and gives the incorrect time, and d combines a using c() and gives the correct time.  Why doesn't c give me the correct answer?

Thanks,

Tim


> a<-as.POSIXct("2000-01-01 12:00:00")
> a
[1] "2000-01-01 12:00:00 HST"

> b<-rbind(a,a)
> b
       [,1]
a 946764000
a 946764000

> c<-as.POSIXct(b,origin="1970-01-01")
> c
[1] "2000-01-01 22:00:00 HST"
[2] "2000-01-01 22:00:00 HST"

> d<-c(a,a)
> d
[1] "2000-01-01 12:00:00 HST"
[2] "2000-01-01 12:00:00 HST"


> structure(0,class="Date")
[1] "1970-01-01"


Tim Clark
Department of Zoology 
University of Hawaii