title: R: Is it possible to update the timezone for all objects in a list that have POSIX-related dates
body: <p>I am exploring Facebook's <code>prophet</code> package in R.  I have noticed that the package sets all dates to the <code>GMT</code> time zone with no way (that I am aware of) to set the time zone when calling the function.</p>
  
  <p>My question is: given that the <code>prophet</code> algorithm returns a list of lists with several different POSIXt or POSIXct objects, is there a way to change all of these objects <em>at once</em> to the same time zone without having to change all objects individually?</p>
  
  <p>For instance, if I have an object created by the <code>prophet</code> function called <code>prophet.model</code>, I can update <code>history.dates</code> or <code>start</code> from the <code>GMT</code> timezone to the <code>"Europe/Paris"</code> time zone using:</p>
  
  <p><code>library(prophet)</code></p>
  
  <p><code>history &lt;- data.frame(ds = seq(as.Date('2015-01-01'), as.Date('2016-01-01'), by = 'd'),
                        y = sin(1:366/200) + rnorm(366)/10)</code></p>
  
  <p><code>prophet.model &lt;- prophet(history)
  </code></p>
  
  <p><code>attr(prophet.model$start,"tzone") &lt;- "Europe/Paris"</code></p>
  
  <p><code>attr(prophet.model$history.dates,"tzone") &lt;- "Europe/Paris"</code></p>
  
  <p>Can this be done instead for the whole object <code>prophet.model</code> at once without having to make individual calls to <code>attr</code>?</p>