title: Excel Days 360 function. - Issue with day count in February
body: <p>The Excel formula below</p>
  
  <pre><code>=DAYS360("1-Jan-2018","31-Jan-2018")
  </code></pre>
  
  <p>returns 30 </p>
  
  <pre><code>=DAYS360("1-Feb-2018","28-Feb-2018")
  </code></pre>
  
  <p>returns 27</p>
  
  <p>I expected both functions to return 30 days as each month is treated as consisting of 30 days. What I am doing wrong or is there a custom function that can give a result of 30 days in both cases? Thanks. </p>