[R] date time problem

Jeff Newmiller jdnewmil at dcn.davis.ca.us
Sun Dec 7 21:59:37 CET 2014

You have not provided a reproducible example, so anything I say could be 
wrong just due to misinterpretation. Please read [1] for suggestions on 
making your examples reproducible, particularly regarding the use of dput 
to provide example data. You have also posted in HTML format, 
which can cause additional scrambling of communications on this list.

>From the notation you are using, I would guess that "Corrected_SA_data" is 
a data frame containing columns "date_time" and "TZ" at a minimum. 
The "date_time" column could be a vector of class POSIXlt or POSIXct... 
except that I cannot reproduce such an object that doesn't print some 
kind of timezone indicator in its output. What version of R are you 
using? Note that such an object contains a tz attribute already, so unless 
you have already made sure that the date_time column knows about that 
timezone, they are probably unrelated to each other.

Note that any POSIXct object is internally represented as a specific 
instant of time. The tz attribute is only used to control how that time 
will be displayed. For example:

testtime3 <- as.POSIXct( rawtime1, tz="America/New_York" )
format( testtime3, tz="Etc/GMT", usetz=TRUE )

If you want the output to omit the timezone information you can omit the 
usetz argument:

format( testtime3, tz="Etc/GMT" )

If you have not yet, you should read [2]. Note that three-letter timezone 
indicators (even though they are given in OUTPUT!) are at best unreliable 
for use in specifying timezones in R... read ?timezones.

[1] http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example
[2] http://www.r-project.org/doc/Rnews/Rnews_2004-1.pdf

On Sun, 7 Dec 2014, Alemu Tadesse wrote:

> Dear R users
> I am puzzled by the following result from R script. I am trying to convert
> local time to UTC time. Time zone is -5, therefore I used the following
> approach.
> Below is the script.
>> Corrected_SA_data$date_time[k-1]
> [1] "2007-03-11 01:00:00"
>> Corrected_SA_data$TZ[k-1]
> [1] -5
>> Corrected_SA_data$date_time[k-1]-Corrected_SA_data$TZ[k-1]*3600
> [1] "2007-03-11 07:00:00 MDT"
> I was expecting this last value to be something like "2007-03-11 06:00:00
> UTC"
> Please correct me if I ma wrong.
> On the other hand I have
>> Corrected_SA_data$date_time[k]
> [1] "2007-03-11 02:00:00"
>> Corrected_SA_data$TZ[k]
> [1] -5
>> Corrected_SA_data$date_time[k]-Corrected_SA_data$TZ[k]*3600
> [1] NA
> I am not sure why I am getting NA.
> Thank you for your help.
> Alemu
> 	[[alternative HTML version deleted]]
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> and provide commented, minimal, self-contained, reproducible code.

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