R help - Apr 2005 - help with POSIX

For the r script below 
>datestr <- "01/01/2004"
>as.POSIXct(as.Date(datestr, "%d/%m/%Y"))
I get the following output
 "2003-12-31 18:00:00 Central Standard Time"

Why is the date a day before. I guess its something to do with the time, but is
there a way to get it to return 2004-01-01 instead?

Thanks in advance...
-Sandeep

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On Fri, 22 Apr 2005, Ghosh, Sandeep wrote:
> For the r script below
>
>> datestr <- "01/01/2004"
>> as.POSIXct(as.Date(datestr, "%d/%m/%Y"))
> I get the following output
> "2003-12-31 18:00:00 Central Standard Time"
>
> Why is the date a day before. I guess its something to do with the time, 
> but is there a way to get it to return 2004-01-01 instead?
Yes.

If you want POSIXt, which includes time as well as date and so depends on 
time zone, use it.
> datestr <- "01/01/2004"
> strptime(datestr, "%d/%m/%Y")
[1] "2004-01-01"
> class(strptime(datestr, "%d/%m/%Y"))
[1] "POSIXt"  "POSIXlt"
> as.POSIXct(strptime(datestr, "%d/%m/%Y"))
[1] "2004-01-01 Pacific Standard Time"

If you just want a date, just use Date
> as.Date(datestr, "%d/%m/%Y")
[1] "2004-01-01"

When you convert from Date to POSIXt it has to pick an arbitrary time 
within the day, and it picks midnight GMT.

 	-thomas

"Ghosh, Sandeep" <sghosh at lexgen.com> writes:
> For the r script below 
> 
> >datestr <- "01/01/2004"
> >as.POSIXct(as.Date(datestr, "%d/%m/%Y"))
> I get the following output
>  "2003-12-31 18:00:00 Central Standard Time"
> 
> Why is the date a day before. I guess its something to do with the
> time, but is there a way to get it to return 2004-01-01 instead?
Just move to Denmark:
> datestr <- "01/01/2004"
> as.POSIXct(as.Date(datestr, "%d/%m/%Y"))
[1] "2004-01-01 01:00:00 CET"

The convention is that Date objects are pegged to 0:00 GMT (not sure
what the rationale is -- ISOdate uses 12:00 GMT -- but Brian probably
thought about it). An easy workaround is to add 12 hours...

-- 
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