RE: ISSUE-17: Advice for representing a duration

> Just to muddy the waters, we also have a Periodic Interval of Time 
(PIVL)

Periodic intervals are things like: "every eight hours for two minutes", 
"every eight hours starting at 4 o'clock" or even "Every eight hours for 
two minutes starting at 4 o'clock".

Obviously, these are more advanced types than the schema built-in duration 
simple types.

pvb