Where to denote the value of a Duration using Schema.org?

Question

I must be missing something extremely simple here but: where do I denote the actual duration value when marking-up an actual Schema.org Duration?

Duration has a couple of properties (all inheriting from Thing), e.g.: name, additionalType, etc. but I was sort of expecting there to be a property called value or something.

From the above link it says the duration is a quantity and should be of format ISO 8601, but where on earth do I put it?

Answer 1

You should put them directly as the value of the Duration property. For example in http://schema.org/Invoice:

 property:billingPeriod
 expected type: Duration 

Setting the billingPeriod from 2014-12-21 with a duration of 30 days in ISO_8601 format, would be 2014-12-21/P30D and in the code:

   <meta itemprop="billingPeriod" content="2014-12-21/P30D" />starts:2014-12-21 30 days

*Example from Invoice schema