title: calculate difference between dates in shell script
body: <p>I want to calculate the difference between two dates in shell script and if the result is greater than three months then it should throw an error to enter the correct start_date.</p>
  
  <p>For example consider: start_date="2016-02-15", end_date=<code>date +%Y-%m-%d</code> </p>
  
  <p>Thanks.</p>