[thredds] NcML Aggregation

Is there a way to promote a global numeric attribute to be a new coordinate variable? 
I read the bottom of
http://www.unidata.ucar.edu/software/netcdf/ncml/v2.2/Cookbook.html
but I am confused (time vs times? and why dateFormatMark which also seems to be creating an ISO 8601 time axis?). 
Can someone provide a more complete, less confusing, working example?

My best guess at the ncml is:

<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
  <variable name="time" type="int" shape="time" >
    <attribute name="units" value="days since 2012-12-31"/>
    <attribute name="_CoordinateAxisType" value="Time" />
  </variable>
  <aggregation dimName="time" type="joinNew" recheckEvery="15 min" >
    <variableAgg name="chla"/>
    <promoteGlobalAttribute name="time" orgName="startDay" />
    <scan location="/testFiles/" suffix=".hdf" subdirs="false" />
  </aggregation>
...

I have tried lots of variations, to no avail.
I have tried lots of variations with joinExisting, with and without the <variable>, to no avail. 
They almost all generate an unhelpful error message like:

Exception in thread "main" java.lang.NullPointerException
at ucar.nc2.ncml.AggregationOuterDimension.promoteGlobalAttributes(AggregationOuterDimension.java:176) at ucar.nc2.ncml.AggregationNew.buildNetcdfDataset(AggregationNew.java:73) 
        at ucar.nc2.ncml.Aggregation.finish(Aggregation.java:404)
        at ucar.nc2.ncml.NcMLReader.readNetcdf(NcMLReader.java:492)
        at ucar.nc2.ncml.NcMLReader.readNcML(NcMLReader.java:437)
        at ucar.nc2.ncml.NcMLReader.access$100(NcMLReader.java:60)
at ucar.nc2.ncml.NcMLReader$NcmlElementReader.open(NcMLReader.java:1406) at ucar.nc2.ncml.NcMLReader$NcmlElementReader.open(NcMLReader.java:1394) at ucar.nc2.dataset.NetcdfDataset.acquireFile(NetcdfDataset.java:625) at ucar.nc2.ncml.Aggregation$Dataset.acquireFile(Aggregation.java:634) at ucar.nc2.ncml.AggregationUnion.buildNetcdfDataset(AggregationUnion.java:65) 
        at ucar.nc2.ncml.Aggregation.finish(Aggregation.java:404)
        at ucar.nc2.ncml.NcMLReader.readNetcdf(NcMLReader.java:492)
        at ucar.nc2.ncml.NcMLReader.readNcML(NcMLReader.java:437)
        at ucar.nc2.ncml.NcMLReader.readNcML(NcMLReader.java:252)
        at ucar.nc2.ncml.NcMLReader.readNcML(NcMLReader.java:202)
at ucar.nc2.dataset.NetcdfDataset.acquireNcml(NetcdfDataset.java:847) at ucar.nc2.dataset.NetcdfDataset.openOrAcquireFile(NetcdfDataset.java:686) 
        at ucar.nc2.dataset.NetcdfDataset.openFile(NetcdfDataset.java:563)
        at ucar.nc2.NCdumpW.print(NCdumpW.java:212)


Any suggestions?

Thank you.

Sincerely,

Bob Simons
IT Specialist
Environmental Research Division
NOAA Southwest Fisheries Science Center
1352 Lighthouse Ave
Pacific Grove, CA 93950-2079
Phone: (831)658-3205
Fax:   (831)648-8440
Email: bob.simons@xxxxxxxx

The contents of this message are mine personally and
do not necessarily reflect any position of the
Government or the National Oceanic and Atmospheric
Administration.
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