diff --git a/Dockerfile b/Dockerfile
index 6db22ac..6785663 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -48,6 +48,9 @@ FROM base AS runner
 WORKDIR /app
 
 ENV NODE_ENV=production
+# Recurring masses/events are built from server-local wall-clock time, so the
+# server must run in the parishes' timezone or times shift by 1h across DST.
+ENV TZ=Europe/Berlin
 
 # Use groupadd and useradd (standard Linux tools) instead of Alpine's syntax
 RUN groupadd --gid 1001 nodejs