title: Displaying time in ascending order from two files
body: <p>I have two files with variable and their respective time. I want to get the output in one dataframe where the variable and time in ascending order.</p>
  
  <pre><code>evt_gts               evt_id
  01-07-2019 16:42:00 976162O
  01-07-2019 16:42:30 976162O
  04-07-2019 15:03:20 976162O
  04-07-2019 15:03:25 976162O
  05-07-2019 10:20:00 976162O
  </code></pre>
  
  <p>The next file is:</p>
  
  <pre><code>timestamp            variable   
  01-07-2019 13:25:03 RefSpd  
  01-07-2019 13:25:10 EffRealized 
  01-07-2019 13:25:30 ABHPosition 
  01-07-2019 13:25:35 LinkVolt    
  01-07-2019 13:25:36 BCPress 
  01-07-2019 23:18:00 speed   
  01-07-2019 23:18:05 temperature 
  01-07-2019 23:31:00 speed   
  01-07-2019 23:31:04 temperature 
  01-07-2019 23:43:00 speed   
  01-07-2019 23:43:05 temperature 
  </code></pre>
  
  <p>Expected Output is:</p>
  
  <pre><code>timestamp     variable
  01-07-2019 13:25:03 RefSpd  
  01-07-2019 13:25:10 EffRealized 
  01-07-2019 13:25:30 ABHPosition 
  01-07-2019 13:25:35 LinkVolt    
  01-07-2019 13:25:36 BCPress 
  01-07-2019 16:42:00 976162O
  01-07-2019 16:42:30 976162O
  01-07-2019 23:18:00 speed   
  01-07-2019 23:18:05 temperature 
  01-07-2019 23:31:00 speed   
  01-07-2019 23:31:04 temperature 
  01-07-2019 23:43:00 speed   
  01-07-2019 23:43:05 temperature 
  04-07-2019 15:03:20 976162O
  04-07-2019 15:03:25 976162O
  05-07-2019 10:20:00 976162O
  </code></pre>