title: filter DATE lesser than (+30 days) than other DATE
body: <p>I have two columns of date (listed in this format 2001-01-01) (DATE1 and DATE2). I want to filter all observations where DATE1(+30 days) &lt; DATE2.</p>
  
  <p>I´ve been trying: filter(DF, (DATE1+30) &lt; DATE2))</p>
  
  <p>but it doesn´t work.</p>
  
  <p>Best regards,
  H</p>