title: Redshift CONVERT_TIMEZONE Issue
body: <p>I am facing an issue with the time conversion. </p>
  
  <p>Whenever I tried to change the time to timestamp. It does not work. 
  I have tried many of functions from <a href="http://docs.amazonaws.cn/en_us/redshift/latest/dg/Date_functions_header.html" rel="nofollow noreferrer">http://docs.amazonaws.cn/en_us/redshift/latest/dg/Date_functions_header.html</a> </p>
  
  <p>But still a single function did not convert the date time to timestamp. An eg. I need date time <code>(2017-12-11 23:38:11)</code> as timestamp <code>(1513036800)</code> format. </p>
  
  <p>Can you please guys tell me the exact function to convert that? </p>
  
  <p>Also I have tried to use <code>CONVERT_TIMEZONE</code> function. 
  It works fine, whenever I passed the static date inside this like: <code>CONVERT_TIMEZONE('GMT','GMT -1','2017-12-11 23:38:11')</code> </p>
  
  <p>But if I change the date with my variable name, I am getting an error message:</p>
  
  <blockquote>
    <p>"ERROR: function convert_timezone("unknown", "unknown", character varying) does not exist HINT: No function matches the given name and argument types. You may need to add explicit type casts." </p>
  </blockquote>
  
  <ul>
  <li>Can you guys tell me function to convert the date into timestamp? </li>
  <li>Can you guys tell me reason of the above error while passing the dynamic variable?</li>
  <li>I want to add timezone and date together and then to get the timestamp value of that, is that possible? </li>
  <li>Dateadd function gives the same error while passing the dynamic variable.</li>
  </ul>