Libyear

A libyear is the calendar time — measured in years — between the release date of the dependency version you use and the release date of the latest stable version. Use a library released in January 2024 when the latest shipped in January 2026, and you are two libyears behind on that dependency. Sum across a project and you get a single freshness figure.

How It Works

The measure was introduced by Cox, Bouwers, van Eekelen, and Visser in "Measuring Dependency Freshness in Software Systems" (ICSE 2015). Its strength is comparability: "three major versions behind" means wildly different things in an ecosystem that ships a major release every month versus every three years, but a year of calendar time means the same everywhere.

Why It Matters

Time-based freshness is the backbone of credible drift measurement. Version distance still adds signal — a skipped major release usually carries breaking changes regardless of timing — which is why drift metrics like DriftScore blend a time term (libyears) with a semver distance term rather than using either alone.

Related Terms

Libyears quantify drift per dependency from lockfile data and anchor the time term in DriftScore.