immich's compose bind-mounts the host /etc/localtime into the app container; NixOS without a set timezone leaves /etc/localtime absent → 'bind source path does not exist: /etc/localtime' → app service rejected (never converges). time.timeZone=UTC creates /etc/localtime (UTC = deterministic CI timestamps). Nix-declared, reversible; helps any recipe binding /etc/localtime. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>