title: how to sum up mm:ss in excel properly - sum() not working?
body: <p>I have selected the format for cells as time. But when applying</p>
  <pre><code>=sum(b1:b40) 
  </code></pre>
  <p>it produces 00:00</p>
  <p><a href="https://i.stack.imgur.com/4f3ea.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4f3ea.png" alt="oo" /></a></p>
  <p>then &quot;custom format&quot; as [h]:mm for the output cell, but no result.
  what am i doing wrong?</p>