title: How many days in ISO8601 duration 'months' and 'years'?
body: <p>I need to represent a duration of 100 days. </p>
  
  <p>From <a href="https://en.wikipedia.org/wiki/ISO_8601#Durations" rel="nofollow">Wikipedia</a>:</p>
  
  <blockquote>
    <p>Durations are represented by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S</p>
  </blockquote>
  
  <p>But for a duration, how many days are in a month? Surely it depends on which month we're in... Ditto, days in a year.</p>