Thanks for your quick confirmation, Craig.  As a workaround, we'll offer our customer a new "Duration" customfield and we'll kindly ask their training administrators to manually fill in this field.  No extra calculations are required. 

Thinking mid-term, I'd say that the availability of the Duration field in the Event datasource will be also useful for many of us. Despite this value can be calculated on the Excel side (manually or via macros) once the report is exported to Excel, it's much more efficient to avoid data manipulation as much as possible. 

Cheers.

Joan