title: Conversion of large dataframe column of string to datetime - unexplainable behavior
body: <p>I have a file containing date and time information as a character string. I want to convert it to a datetime POSIXlt object and to that end I use the strftime() base function in R.</p>
  
  <p>While the command works fine when I am passing it a character string, or when I apply it a small slice of the dataframe, when I apply it in a larger slice or in the entire datadrame it fails to capture the Hour. Please see below:</p>
  
  <pre><code>d = "2017-11-18 01:00:00"
  t = strftime(d, format = "%Y-%m-%d %H:%M")
  t
  '2017-11-18 01:00'
  
  head(data %&gt;% dplyr::slice(1:1000) %&gt;% mutate(DateTime1 = strftime(DateTime, format = "%Y-%m-%d %H:%M")))
  DateTime    Junction    Vehicles    ID  DateTime1
  2015-11-01 00:00:00 1   15  20151101001 2015-11-01 00:00
  2015-11-01 01:00:00 1   13  20151101011 2015-11-01 01:00
  2015-11-01 02:00:00 1   10  20151101021 2015-11-01 02:00
  2015-11-01 03:00:00 1   7   20151101031 2015-11-01 03:00
  2015-11-01 04:00:00 1   9   20151101041 2015-11-01 04:00
  2015-11-01 05:00:00 1   6   20151101051 2015-11-01 05:00
  
  head(data %&gt;% mutate(DateTime1 = strftime(DateTime, format = "%Y-%m-%d %H:%M")))
  DateTime    Junction    Vehicles    ID  DateTime1
  2015-11-01 00:00:00 1   15  20151101001 2015-11-01 00:00
  2015-11-01 01:00:00 1   13  20151101011 2015-11-01 00:00
  2015-11-01 02:00:00 1   10  20151101021 2015-11-01 00:00
  2015-11-01 03:00:00 1   7   20151101031 2015-11-01 00:00
  2015-11-01 04:00:00 1   9   20151101041 2015-11-01 00:00
  2015-11-01 05:00:00 1   6   20151101051 2015-11-01 00:00
  </code></pre>
  
  <p>How this erratic behavior is to be explained and how can I convert the datetime column for the entire dataset?</p>
  
  <p>Your advice will be appreciated.</p>