title: Discrepancy between date-fns startOfWeek and format W
body: <p>I'm having a problem with <code>date-fns</code> and the way it handles "start of weeks".</p>
  
  <p>The "start of week" of 2015-01-01, according to my calendar, was 2014-12-29.</p>
  
  <p>But, <code>date-fns#startOfWeek</code> tells me otherwise:</p>
  
  <pre><code>❯❯❯ dateFns.startOfWeek(new Date('2015-01-01'))
  Sun Dec 28 2014 00:00:00 GMT+0100 (Central European Standard Time)
  </code></pre>
  
  <p>If I format the result of <code>startOfWeek</code> (2014-12-28) to just return the week number, what I get is "52" (which doesn't match with the result provided by <code>dateFns#startOfWeek</code>):</p>
  
  <pre><code>❯❯❯ dateFns.format(new Date('2014-12-28'), 'W')
  "52"
  </code></pre>
  
  <p>But, if I format the original date (2015-01-01), it will return "1":</p>
  
  <pre><code>❯❯❯ dateFns.format(new Date('2015-01-01'), 'W')
  "1"
  </code></pre>
  
  <p>Why is there this discrepancy? What's the proper way to get the right "start of week" date and week number?</p>
  
  <p>Note, I'm setting <code>TZ=Etc/UTC</code> in my environment variables when I run the above commands on my Node.js CLI. So it shouldn't be a timezone issue.</p>