The relase timeline must be a balance between the need to support additional requirements and provide guidances that improves interoperability and the need to give implementers sufficient time to adapt. Once FHIR content becomes normative, there will not be breaking changes within a resource. Even prior to going normative, the FHIR Maturity Model ensures that breaking change proposals for "mature" resources face increased scrutiny. The specific timelines will evolve based on implementer need. The timelines documented in the specification are to set implementer expectations and, at present, the FMG considers them to be a reasonable expectation to set.