title: Error in mutate_impl(.data, dots) : Evaluation error: non-numeric argument to binary operator
body: <p>When I am trying to run following function : </p>
  
  <pre><code>foo &lt;- air_reserve %&gt;%
    mutate(reserve_date = date(reserve_datetime),
           reserve_hour = hour(reserve_datetime),
           reserve_wday = wday(reserve_datetime, label = TRUE),
           visit_date = date(visit_datetime),
           visit_hour = hour(visit_datetime),
           visit_wday = wday(visit_datetime, label = TRUE),
           diff_hour = time_length(visit_datetime - reserve_datetime, unit = "hour"),
           diff_day = time_length(visit_datetime - reserve_datetime, unit = "day")
    )
  </code></pre>
  
  <p>I am getting following error: </p>
  
  <pre><code>Error in mutate_impl(.data, dots) : 
    Evaluation error: non-numeric argument to binary operator.
  </code></pre>
  
  <p>How can I solve this issue ?</p>
  
  <h1>Note:</h1>
  
  <p><code>foo</code> function is taken from this <a href="https://www.kaggle.io/svf/1834414/0e00324611a5d00958c6b418517570d0/__results__.html#" rel="nofollow noreferrer">link</a></p>