[thredds] timeCoverage with start, end, and duration

[Doug Lindholm <doug.lindholm@xxxxxxxxxxxxxxxxx>]

I notice that the THREDDS catalog schema (1.0.2) has maxOccurs="3" for 
these elements of timeCoverageType. What does it mean when start, end, 
and duration are all set?

I'm wondering if it could support the following use case:
I have a dataset that is updated regularly but published one week after 
the measurement was made. Thus the end of my time coverage is "7 days ago".

I gather that I could define a start time of "7 days ago" by specifying 
end="present" and duration="7days". But if I also specify a fixed date 
for the start time, is it in any way acceptable to interpret the 
duration as an offset from "present"? Any better ideas?

Thanks,
Doug