title: orgmode logbook drawer total time not calculated as expected
body: <p>I use the logbook drawer in a headline like this:</p>
  
  <pre><code>* WIP My Project
    :LOGBOOK:
    CLOCK: [2020-01-15 mer. 10:18]--[2020-01-15 mer. 11:18] =&gt;  1:00
    CLOCK: [2020-01-14 mar. 10:12]--[2020-01-14 mar. 11:12] =&gt;  1:00
    CLOCK: [2019-11-13 mer. 14:20]--[2019-11-13 mer. 14:50] =&gt;  0:30
    :END:
  </code></pre>
  
  <p>When I ask for the total time (pressing C-c C-x C-d) I have 2:00 and not 2:30 as expected. The CLOCK entry of 2019 is excluded. I wonder why...
  Any idea? Thanks in advance! </p>