title: How to place dates on the same scale using a reference point
body: <p>I have a set of dates for multiple years and I am wondering how to place them on the same scale using a reference point. </p>
  
  <p>For example, I have the following dates:</p>
  
  <p>"2018-04-15" "2018-04-30" "2018-05-06" "2018-05-12" "2018-05-13"</p>
  
  <p>I want to create a separate column that counts the number of days these dates are from </p>
  
  <p>"2018-11-06". </p>
  
  <p>Thanks so much!</p>