title: unused argument date in R
body: <p>I am getting an odd error when I try to extract the year from a date object</p>
  
  <p>here is a dput of my dates:</p>
  
  <pre><code>structure(list(date = structure(c(15706, 15707, 15708, 15709, 
  15710, 15711), class = "Date")), .Names = "date", row.names = c(NA, 
  -6L), class = c("tbl_df", "tbl", "data.frame"))
  </code></pre>
  
  <p>when I pipe to <code>lubridate::year(date)</code> I get the following error.</p>
  
  <pre><code>Error in year(., date) : unused argument (date)
  </code></pre>