title: symbol to use for minus `1 month` in `dayTimeDuration` from `dateTime`
body: <p>Anyone knows what symbol to use for minus <code>1 month</code> in <code>dayTimeDuration</code> from <code>dateTime</code>? </p>
  
  <p><code>xs:dayTimeDuration('-P30D')</code> subtracts 30 days which could indirectly be a month. But, in the cases where feb has 28 days, will this still be accurate?</p>
  
  <p><a href="http://www.datypic.com/sc/xsd/t-xsd_dayTimeDuration.html" rel="nofollow">http://www.datypic.com/sc/xsd/t-xsd_dayTimeDuration.html</a></p>