diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5386e49..1cbfc5c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -177,6 +177,8 @@ jobs:
       # Force in-memory rate limiter to avoid cross-test state when Redis drops.
       # Redis fallback downgrades to max/10 limits which rate-limits unit tests.
       RATE_LIMIT_BACKEND: memory
+      # Tests assume Europe/Berlin for month-boundary math (new Date(y,m,1)).
+      TZ: Europe/Berlin
       NEXTAUTH_URL: ${{ env.CI_AUTH_URL }}
       AUTH_URL: ${{ env.CI_AUTH_URL }}
       NEXTAUTH_SECRET: ${{ env.CI_AUTH_SECRET }}