this is to follow up after a initial proposal by @brianmaissy in #4760 and #4736

key issues to decide on/highlight

• new internal helper classes for date times and time deltas
• meaning of relative for time-deltas
• decision on whether timestamps can have a meaningful rel at all ( i currently lean on no)
• documentation of the explicit requirements for declaring approximate date-times in nested structures (no implication as we don't have sane defaults for abs/rel)