title: Can't Extract Date from Timestamp in BigQuery
body: <p>I'm trying to get all rows associated with a particular entity ID over the last week. Each row has a timestamp. I'd like to group those entities by day, by extracting the date from the datetime timestamp, but when I try, I get</p>
  
  <blockquote>
    <p>Error: (L6:28): expression STRFTIME_UTC_USEC([data_timestamp], '%Y-%m-%d') in GROUP BY is invalid</p>
  </blockquote>
  
  <p>from bigquery. It appears as though something is wrong with the DATE() function. </p>
  
  <p>Here is the full query I am running:</p>
  
  <pre><code>SELECT Count(*) FROM [myproj:mydataset.mytable] 
  WHERE 
      bool_property=False 
  AND 
      entity_id=5667423172689920 
  AND
      DATEDIFF(CURRENT_DATE(), data_timestamp) &lt;= 7
  GROUP BY DATE(data_timestamp)
  </code></pre>
  
  <p>Looking at the data, it would appear that the quality, here called 'data_timestamp' is in fact a proper timestamp; I really don't understand why the DATE() function would be failing.  </p>
  
  <p>Any help or tips? Thanks!</p>