title: create another dataframe datetime column based on the value of the datetime in another dataframe column
body: <p>I have a dataframe which has a datetime column lets call it my_dates.</p>
  <p>I also have a list of dates which has say 5 dates for this example.</p>
  <pre><code>15th Jan 2020
  20th Mar 2020
  28th Jun 2020
  20th Jul 2020
  8th Aug 2020
  </code></pre>
  <p>What I want to do is create another column in my datframe where it looks at the datetime in my_dates column &amp; where it is less than a date in my date list for it to take that value.</p>
  <p>For example lets say for this example say its 23rd June 2020. I want the new column to have the value for this row of 28th June 2020. Hopefully the examples below are clear.</p>
  <pre><code> More examples
  
   my_dates             expected_values
  
   14th Jan 2020        15th Jan 2020
   15th Jan 2020        15th Jan 2020
   16th Jan 2020        20th Mar 2020
   ...                  ...
   19th Mar 2020        20th Mar 2020
   20th Mar 2020        20th Mar 2020
   21st Mar 2020        28th Jun 2020
  </code></pre>
  <p>What is the most efficient way to do this rather than looping?</p>