[R] DateTime Math in R - POSIXct

[David Winsemius]

On May 30, 2011, at 10:20 PM, Galen Moore wrote:

> Greetings -
>
>
>
> I'm battling POSIXct, as per the code below.  My input is actually  
> an XL
> file, but the weird results below correctly model what I am seeing  
> in my
> program.
>
>
>
> Before I punt and use lubridate or timeDate, could anyone please  
> help me
> understand why POSIXct forces my variable back to GMT?
>
>
>
> I suspect that I'm not properly coding the tzone value, but it does  
> not
> throw an error as-is.
>
>
>
>
>
>> tstamp <- "2011-05-22 11:45:00 MDT"
>
>> mode(tstamp)
>
> [1] "character"
>
>>
>
>> dateP <- as.POSIXct(tstamp, origin="1970-01-01", tzone="MDT")
>
>> mode(dateP)
>
> [1] "numeric"
>
>> dateP
>
> [1] "2011-05-22 11:45:00 MDT"
>
>>
>
>> dateN <- as.numeric(dateP)
>
>> dateN
>
> [1] 1306086300
>

So now the internal representation is referenced to GMT

>>
>
>> dateP2 <- as.POSIXct(dateN, origin="1970-01-01", tzone="MDT")
>
>> dateP2

And you are telling the function to add as many hours offset as needed  
for the difference between GMT and MD T.....

-- 
David.
>
> [1] "2011-05-22 18:45:00 MDT"
>

David Winsemius, MD
West Hartford, CT