From simon.urbanek at r-project.org  Fri Jan 11 19:27:15 2013
From: simon.urbanek at r-project.org (Simon Urbanek)
Date: Fri, 11 Jan 2013 13:27:15 -0500
Subject: [Rd] Date time conversion bug (as.POSIXct)?
In-Reply-To: <1357899135306-4655215.post@n4.nabble.com>
References: <1357899135306-4655215.post@n4.nabble.com>
Message-ID: <4F88DD6E-1BC4-4639-8A46-F591BD0AB26A@r-project.org>

On Jan 11, 2013, at 5:12 AM, Dries Verheyen wrote:

> There is something wrong, I think, with the date-time conversion from a
> numeric value if you use Central European Time (CET) as timezone.
> Examples from R:
> 
> If I use the GMT time zone it is OK, I get the same time back from
> as.POSIXct as I entered
> 
>> as.POSIXct(as.numeric(strptime("30/01/2012 13:00:00", format="%d/%m/%Y
>> %H:%M:%S",tz="GMT")),origin="1970-01-01",tz="GMT")
> 
> [1] "2012-01-30 13:00:00 GMT"
> 
> If I go from CET to GMT there is also no problem:
> 
>> as.POSIXct(as.numeric(strptime("30/01/2012 13:00:00", format="%d/%m/%Y
> %H:%M:%S",tz="CET")),origin="1970-01-01",tz="GMT")
> 
> [1] "2012-01-30 12:00:00 GMT"
> 
> In winter we are 1 hour ahead of Greenwich.
> 
> Also in summer it is OK (2 hours ahead of Greenwich):
> 
>> as.POSIXct(as.numeric(strptime("30/05/2012 13:00:00", format="%d/%m/%Y
> %H:%M:%S",tz="CET")),origin="1970-01-01",tz="GMT")
> 
> [1] "2012-05-30 11:00:00 GMT"
> 
> The problem occurs when I use both the same time zone, if it is not GMT:
> 
>> as.POSIXct(as.numeric(strptime("30/01/2012 13:00:00", format="%d/%m/%Y
> %H:%M:%S",tz="CET")),origin="1970-01-01",tz="CET")
> 
> [1] "2012-01-30 12:00:00 CET"
> 
>> as.POSIXct(as.numeric(strptime("30/05/2012 13:00:00", format="%d/%m/%Y
> %H:%M:%S",tz="CET")),origin="1970-01-01",tz="CET")
> 
> [1] "2012-05-30 12:00:00 CEST"
> 
>> as.POSIXct(as.numeric(strptime("30/05/2012 13:00:00", format="%d/%m/%Y
>> %H:%M:%S",tz="EST")),origin="1970-01-01",tz="EST")
> [1] "2012-05-30 18:00:00 EST"
> 
> It looks like the function just puts the name of the time zone behind the
> time calculated in GMT, and does not calculate the time in the asked time
> zone.

It works perfectly fine the way it's supposed to - you just give it wrong input. What you get from as.numeric(...) is the offset from the epoch (1970-01-01 GMT!) yet you are then forcing as.POSIXct to interpret it as offset from the local time zone which is obviously wrong. I suppose what you really wanted is

> .POSIXct(as.numeric(strptime("30/05/2012 13:00:00", format="%d/%m/%Y%H:%M:%S",tz="EST")),"EST")
[1] "2012-05-30 13:00:00 EST"

The problem in your case is that you want to have origin in a different timezone than the result which is a bit convoluted.

Cheers,
Simon




> Is it just how the function works? because for me it is a bit confusing like
> that.
> 
> 
> 
> --
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> 
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[Rd] Date time conversion bug (as.POSIXct)?
Simon Urbanek