title: parse a string date in particular format, fail otherwise
body: <p>Yes, I know that the following is not best practice, but inherited db.</p>
  
  <p>I have string as 'MM/dd/yy tt:hh:mm'. At times string is 'tt:hh:mm' in error.</p>
  
  <p>If I cast 'tt:hh:mm' to datetime, it succeeds, and puts in default date as 1900-01-01. I would want this to fail.</p>
  
  <p>isdate(' 08:00:00') success returns 1.</p>
  
  <p>I would want the following:</p>
  
  <pre><code>myisdatetime(' 08:00:00') fails return 0
  myisdatetime( 'MM/dd/yy tt:hh:mm') success returns 1
  </code></pre>
  
  <p>Maybe even specify datetime format. i.e. Parse exact.</p>