Add ADR about use of CalVer for main project releases

docs/adr/0013-ssvc-project-versions.md

@@ -0,0 +1,71 @@
+---
+status: "accepted"
+date: 2025-09-15
+---
+# SSVC Project Versions Follow Calendar Versioning (CalVer)
+
+## Context
+
+Individual SSVC objects (decision points, tables, etc.) use Semantic Versioning (SemVer).  
+For the overall SSVC project and documentation, we need a scheme that better reflects frequent, incremental updates and communicates recency to users.
+
+## Decision Drivers
+
+- Object-level changes use SemVer, but the project evolves more fluidly.  
+- Documentation often changes without altering underlying objects.  
+- Users need clear signals about how current the materials are.
+
+## Options
+
+- **CalVer** for project and documentation  
+- **SemVer** for project and documentation (status quo)
+
+## Decision
+
+**Chosen:** CalVer (`YYYY.MM.patch`)  
+
+- **Major (`YYYY`) and minor (`MM`)** = release year and month  
+  - First significant update in a month → `YYYY.M.0` (e.g., June 2025 → `2025.6.0`)  
+- **Patch** = subsequent updates in the same month or smaller corrections  
+
+**Significant updates** include:  
+- Adding or restructuring sections in ways that affect usability  
+- Adding/revising decision points, tables, or other SSVC objects  
+- Adding features that change how SSVC is applied  
+
+We expect ~1–4 `YYYY.MM` releases per year, with patches as needed.  
+Months use single digits (`2025.6`) to keep versions concise.
+
+## Rationale
+
+CalVer suits SSVC’s character as a living framework:  
+- Clearly signals recency (date in the version number)  
+- Fits both documentation-focused and object-focused updates  
+- Avoids SemVer debates over what counts as “major” or “minor”  
+
+Individual SSVC objects will continue to use SemVer for backward compatibility.  
+
+PyPI releases are expected to follow a similar CalVer scheme, but may include higher-resolution
+date-time stamps for individual builds (e.g., `2025.6.141053`).
+
+## Consequences
+
+- **Good:** Versions clearly indicate recency and distinguish project vs. object evolution  
+- **Neutral:** CalVer conveys less about compatibility, but SemVer at the object level mitigates this  
+- **Bad:** Breaks continuity with past project-level SemVer (e.g., SSVC 1.x, 2.x)  
+
+## Confirmation
+
+The CalVer scheme will be applied via Git tags and GitHub releases.  
+
+## Alternatives Rejected
+
+**Continue SemVer for project/docs**  
+- **Good:** Maintains continuity  
+- **Bad:** Cannot easily express documentation updates independent of object versions  
+
+## References
+
+- [SSVC Decision Point Versioning Rules](0003-ssvc-decision-point-versioning-rules.md)
+- [CalVer](https://calver.org/)  
+- [SemVer](https://semver.org/)  

docs/adr/index.md

@@ -24,6 +24,7 @@ the decision records that have been made.
 - [0010 - Outcome Sets are separate from Decision Point Groups](0010-outcome-sets-are-separate-from-decision-point-groups.md)
 - [0011 - Correspondence between Automatable v2.0.0, Value Density v1.0.0, and CVSS v4](0011-automatable-and-value-density-and-CVSSv4.md)
 - [0012 - SSVC Namespaces](0012-ssvc-namespaces.md)
+- [0013 - SSVC Project Versions](0013-ssvc-project-versions.md)
 
 ## Rejected Records