title: Calculate total number of values per day with pandas
body: <p><strong>I have the following data frame</strong></p>
  
  <pre><code>       UNIT  C/A   DATETIME                TOTAL COUNTs
         R248  HOO7  2018-03-03 03:00:00     139.0
                     2018-03-03 07:00:00     927.0
                     2018-03-04 11:00:00    1946.0
                     2018-03-05 07:00:00    1330.0
                     2018-03-05 11:00:00    2358.0
  </code></pre>
  
  <p>I want to calculate the sum of 'TOTAL COUNTS' for each day.So I want the dataframe to look like this:</p>
  
  <pre><code>        UNIT  C/A   DATETIME     TOTAL COUNTs
         R248  HOO7  2018-03-03    1066
                     2018-03-04    1946
                     2018-03-05    3688
  </code></pre>
  
  <p>I read the I have to set the index of this dataframe to 'datetime' index and then use .resample, but I am not sure how to do so.</p>