title: Replace certain dates with NA
body: <p>I'm trying to replace certain dates with NA.</p>
  
  <p>I tried the following but it did not work.</p>
  
  <pre><code>df &lt;- data.frame(dates = seq.Date(as.Date("1910-01-01"), as.Date("1999-01-01"), "days")) %&gt;%
         mutate_if(dates &lt; as.Date("1990-01-05"),NA)
  </code></pre>
  
  <p>Does anyone know how to do this?</p>
  
  <p>Thanks!</p>