title: timestamp string (Unix time) to datetime or pandas.Timestamp
body: <p>From a source I retrieve some data in JSON format. I want to save this data (measurements in time) as a text file. Repeatedly I want to go the same source and see if new measurements are available, if so I want to add it to the other measurements.</p>
  
  <p>The data I get looks like this:</p>
  
  <pre><code>{"xyz":[{"unixtime":"1458255600","time":"00:00","day":"18\/03","value":"11","paramlabel":"30-500 mHz","popupcorr":"550","iconnr":"7","paramname":"30-500 mHz"},{"unixtime":"1458256200","time":"00:10","day":"18\/03","value":"14","paramlabel":"30-500 mHz","popupcorr":"550","iconnr":"7","paramname":"30-500 mHz"},etc.]}
  </code></pre>
  
  <p>I load this data into a pandas DataFrame to be able to work with it more easily. When I load this into a dataframe however, all columns are treated as strings. How can I make sure that the unixtime column is treated as a timestamp (such that I can convert to a datetime)?</p>