Calculated Field in Mapping Object

We know about calculated fields that are using aggregation functions, for measures...but what if we have this scenario:

• we are not in control of the web service that provides json data, so we cannot modify/add fields to it on the server (think something like a public service, not ours);
• we have two fields arriving in the json stream, date_started, and date_ended;
• we want to calculate days_diff as the number of days between the two dates, and aggregate min_days_diff, max_days_diff, avg_days_diff;

Is that possible? This sounds like a feature any reporting engine should have and, to be frank with you, as I am pretty sure you are reading each node in the json to modify caption, data type, etc., should be easily implemented as a mapping object feature. I read one of your answers where another field could be referenced as a property inside the mapping config of a field. Can that functionality be expanded with a function that executes for each property mapping?
Thank you in advance,
Serban
PS. I understand customizeCell can alter a field value but that only triggers for visible cells, that lazy load into screen, and would not participate in aggregations. Unless I am missing something, that is not a solution to my question.