title: Issues in getting week numbers with weeks starting on Sunday in Python?
body: <p>I am using Python 3.6 and having issues getting week numbers starting from Sunday.</p>
  
  <p>How can I find week numbers with weeks starting on Sunday in Python?</p>
  
  <pre><code>t1 = datetime.datetime.now()
  
  t1.strftime("%U")
  </code></pre>
  
  <p>For example, on <code>09/16/2018 Sunday</code> it should give <code>WW38</code> and not <code>WW37</code>, which it is giving now.</p>
  
  <p>I see that both <code>"%U"</code>, <code>"%V"</code> and <code>"%W"</code> are giving the same. </p>
  
  <p>Any help will be appreciated. </p>
  
  <p>A small cut of the dataframe is as below:</p>
  
  <pre><code>   time                   value
  2018/09/15 1:08:19 PM   11.87
  2018/09/16 6:23:17 PM   10.794
  2018/09/16 6:37:48 PM   10.313
  2018/09/16 6:54:14 PM   10.578
  2018/09/16 6:58:24 PM   11.057
  2018/09/19 9:08:24 PM   13.09
  </code></pre>
  
  <p>For each of the time I am creating a WW column.. The issue is that I want the WW to start on Sunday and not Monday.</p>
  
  <p>My code:</p>
  
  <pre><code>ww_l =[]
  for date in df['time']:  
    date_v = datetime.strptime(tv, "%Y-%m-%d %H:%M:%S")  
    curr_ww = datetime.date(date_v).strftime("%W")   
    ww_l.append(curr_ww)
  df['WW'] = pd.DataFrame(ww_l)
  </code></pre>