title: Boolean expression in CASE in WHERE clause does not work
body: <p>I am having problems with Firebird SQL statement in version Firebird 2.5.</p>
  <p>Based on today's date, I have to select either this month's data or the previous month's data.</p>
  <pre><code>SELECT * FROM FA_DOBAVNICA
  WHERE
  1=1
  
  AND CASE WHEN
      extract(day from cast('Now' as date)) &lt; 9
  THEN
     DATUM_NAROCILA BETWEEN 'start of previous month' AND 'end of previous month'
  ELSE
     DATUM_NAROCILA BETWEEN 'start of this month' AND 'end of this month'
  END 
  </code></pre>
  <p>I am getting a <code>104</code> error <code>Token unknown for BETWEEN</code>. I have no idea what am I doing wrong.</p>