title: Javascript function, need to decrement every 30 secs time elapsed
body: <p>I'm trying to make the <code>this.SPAWN_FREQUENCY</code> decrease by 200 after time elapsed reaches 30 seconds, 1 min, 2 min and so on. Basically levels in the game increasing the amount of enemies to defeat. I've tried a load of code and still can't get anything to work.<br>
  Any suggestions where to start?<br>
  Sorry if I didn't provide enough detail.</p>
  
  <pre><code> function EnemyManager (timer) {
         this.enemies = [];
         this.SPEED = -5;
         this.SPAWN_FREQUENCY = 1300;
         this.spawnTimer = 0;
  </code></pre>
  
  <p><em>Do I add 'if' statement?</em></p>