The motivation for this request is to allow the calculation of a person's current age, like "now() - Patient.birthDate". Currently section "6.7. Date/Time Arithmetic" requires that "the right operand must be a quantity". The request here is to allow the right operand to be another DateTime, if the left operand is also a DateTime. (I have not thought about and don't have an opinion about what to do for other possibile combinations of Date, Time, and DateTime).

My suggestion would be to have the result of the subtraction be a quantity with "ms" units.