title: How can I write a C #if directive that branches based on the current date
body: <p>What I want is something functionally like this:</p>
  
  <pre><code>#if TODAY&lt;OCTOBER_31_2017
  #define OK 0
  #elif TODAY==OCTOBER_31_2017     
  #define OK 1
  #else     
  #define OK 2
  #endif
  </code></pre>
  
  <p>So I want "OK" to have the relevant compile-time value, depending on today's date.</p>
  
  <p>Is there any way to do it? </p>