title: replace NA in date column with blank
body: <p>I have a date column in my dataframe with some NA values. I am trying to replace this NA values with blank using the command, <code>df$FirstDate[is.na(df$FirstDate)] &lt;- " "</code><br>
  I am getting an error </p>
  
  <blockquote>
    <p>Error in charToDate(x) : character string is not in a standard unambiguous format</p>
  </blockquote>
  
  <p>Not sure how to fix this error. Any help on this topic is much appreciated.</p>
  
  <p>Here's the <code>dput</code> output from date column</p>
  
  <pre><code>df = structure(c(NA_real_, NA_real_, NA_real_, NA_real_, NA_real_, NA_real_), class = "Date")
  </code></pre>