title: how to check a given string is a timezone instance
body: <p>how can i check a <code>given string is a timezone instance</code> , say i have argument value namely 'date' in a method, i want check it as a timezone instance or timezone aware instance before return it, i have trid the following, but it failed</p>
  
  <pre><code>def get_date(date=None):
     if not isinstance(date, timezone):
         raise ValidationError('please provide timezone aware date type value')
  </code></pre>
  
  <p>it shows the error,</p>
  
  <pre><code>TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types  
  </code></pre>
  
  <p>how can check a string value as timezone aware date instance?</p>