title: validate date in unix for DD-MM-YYYY format
body: <p>How to validate the date in <code>Unix</code>.<br>
  Where the date format is <code>DD-MM-YYYY</code>.<br>
  For example if i run one script <code>./ValidateDate.sh DD-MM-YYYY</code> then it should check that the date is in this format and also that the date exists.<br>
  If date is valid and in above format then proceed for next else print message 'Invalid date'</p>
  
  <p>Thanks.</p>