Updated description of billingPeriod

[RichardWallis]

Added invoiceDate property
Updated examples to reflect changes
Fixes #2210

[danbri]

rdfs:comment "The time interval used to compute the invoice [[paymentDueDate]]." .

Adding a reference to paymentDueDate doesn't make a sentence

[RichardWallis]

Updated wording for billingPeriod to:

The time interval used to compute the [[paymentDueDate]] for the invoice.

[dr-shorthair]

Terminology is still jumbled here.

• time-interval = an interval on the time-line with a beginning and end
• duration = distance or amount of time between two instants or temporal positions
• period = usually synonym for time-interval

So I would suggest that either the property name or the range needs to be changed:

• billingPeriodDuration could have range Duration
• billingPeriod should have range Interval or Period, which in turn should have a intervalDuration or periodDuration property whose range is Duration
  But the proposed state is not coherent.

[github-actions]

This pull request is being tagged as Stale due to inactivity.