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    <p>Timezone names are not as simple as you would like. See <a href="http://en.wikipedia.org/wiki/Tz_database" rel="nofollow">http://en.wikipedia.org/wiki/Tz_database</a> for background and <a href="http://en.wikipedia.org/wiki/List_of_tz_database_time_zones" rel="nofollow">http://en.wikipedia.org/wiki/List_of_tz_database_time_zones</a> for a list of the names that are used. By far the best thing is to use the <code>tz = 'country / city'</code> notation and to explicitly set the time zone of the local system.</p> <p>So, here's a script that uses two different methods to encode the time zone:</p> <pre><code>Sys.setenv(TZ='GMT') pst.abr &lt;- as.POSIXct('2011-01-10 06:45:00', tz = 'PST') est.abr &lt;- as.POSIXct('2011-01-10 09:45:00', tz = 'EST') pst.country.city &lt;- as.POSIXct('2011-01-10 06:45:00', tz = 'America/Los_Angeles') est.country.city &lt;- as.POSIXct('2011-01-10 09:45:00', tz = 'America/New_York') </code></pre> <p>If we look at the POSIXct values that we would have like to have been PST, we see that they actually have two different values. Starting with the abbreviation (<code>tz ='PST'</code>), you get this:</p> <pre><code>&gt; pst.abr [1] "2011-01-10 06:45:00 UTC" &gt; as.numeric(pst.abr) [1] 1294641900 </code></pre> <p>You see that the data we defined using <code>tz='PST'</code> isn't actually in the PST timezone, but has inherited the system's timezone. </p> <p>Compare this to the data we defined using the country\city:</p> <pre><code>&gt; as.numeric(pst.country.city) [1] 1294670700 &gt; pst.country.city [1] "2011-01-10 06:45:00 PST" </code></pre> <p>So, only the data that we explicitly encode with country/city information has the correct timezone information.</p>
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    1. USAndy Clifton
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    1. COWhen I think about it, this is really odd, because the stuff I encoded using country/city notation has now got the correct abbreviation. POSIXct does my head in.
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    2. COthanks. that did it. originally I used 'US/Eastern' 'US/Pacific', but since it kept printing as EST/PST, at some point, I inadvertently switched to those without thinking about it too much.
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