in a layout with many portals what is the cleanest way to calc an overall modification date based on the latest date of all related sections.

If your relationships are sorted by modification date descending,

Max( relone::moddate; reltwo::moddate; ... ; reln::moddate ),

if ascending,

Max( Last(relone::moddate); Last(reltwo::moddate); ... ; Last(reln::moddate) ),

if neither,

Max( Max(relone::moddate); Max(reltwo::moddate); ... ; Max(reln::moddate) ),

or an appropriate combination of the three.

If you want to include the parent record, just add the current table's moddate to the Max test.