title: How to find best time a person opens email
body: <p>I have a dataset that contains details as follows:</p>
  
  <pre><code>campaignID, emailAddress, sendDatetime, openDatetime
  A           a@x.com       20151001-1000 20151001-1005
  A           b@y.com       20151001-1000 20151001-1230
  B           a@x.com       20151007-1000 20151007-1100
  B           b@y.com       20151007-1000 20151007-1235
  </code></pre>
  
  <p>Using R, how can I find the earliest open time for each individual email address? The output should be:</p>
  
  <pre><code>emailAddress, openTime
  a@x.com,      10
  b@z.com,      12
  </code></pre>