Unlike the balance on an account, this is intended to be used as the actual value, and therefore not something that continually changes. The short description of the length property will be changed to explicitly include "actual":  "Actual quantity of time the encounter lasted (less time absent)" Additional clarification text will be added to the length property to describe that when missing it is the time in between the start and end values. If the precision on these values is low (e.g. to the day only) then this may be considered was an all day (or multi-day) encounter, unless the duration is included, where that amount of time occurred sometime during the interval. This will have taken into consideration any leave of absence or other non-encounter time. v2 mapping for length will be updated to PV2-11 (which is actual length in days) Add an invariant to ensure that the length is less than the period (if computable). The estimated length would come from the appointment, or from a new standard extension for the estimated duration (comparable to HL7v2 PV2-10).