title: python pandas extract unique dates from time series
body: <p>I have a DataFrame which contains a lot of intraday data, the DataFrame has several days of data, dates are not continuous. </p>
  
  <pre><code> 2012-10-08 07:12:22            0.0    0          0  2315.6    0     0.0    0
   2012-10-08 09:14:00         2306.4   20  326586240  2306.4  472  2306.8    4
   2012-10-08 09:15:00         2306.8   34  249805440  2306.8  361  2308.0   26
   2012-10-08 09:15:01         2308.0    1   53309040  2307.4   77  2308.6    9
   2012-10-08 09:15:01.500000  2308.2    1  124630140  2307.0  180  2308.4    1
   2012-10-08 09:15:02         2307.0    5   85846260  2308.2  124  2308.0    9
   2012-10-08 09:15:02.500000  2307.0    3  128073540  2307.0  185  2307.6   11
   ......
   2012-10-10 07:19:30            0.0    0          0  2276.6    0     0.0    0
   2012-10-10 09:14:00         2283.2   80   98634240  2283.2  144  2283.4    1
   2012-10-10 09:15:00         2285.2   18  126814260  2285.2  185  2285.6    3
   2012-10-10 09:15:01         2285.8    6   98719560  2286.8  144  2287.0   25
   2012-10-10 09:15:01.500000  2287.0   36  144759420  2288.8  211  2289.0    4
   2012-10-10 09:15:02         2287.4    6  109829280  2287.4  160  2288.6    5
   ......
  </code></pre>
  
  <p>How can I extract the unique date in the datetime format from the above DataFrame? To have result like <code>[2012-10-08, 2012-10-10]</code></p>