title: Conditional formatting conundrum based off two dates in Google sheets
body: <p>I am using Google Sheets.
  I have a lot of columns. The only ones that count are: a column with dd/mm/yyyy named "start date" and a column with dd/mm/yyyy named "finish date".
  I want to:</p>
  
  <ol>
  <li>highlight red if there is a start date but no finish date.</li>
  <li>highlight orange if there is a finish date but no start date.</li>
  </ol>
  
  <p>I haven't been able to get a formula (in conditional formatting) to work.</p>