diff --git a/src/atm/main.py b/src/atm/main.py
index 8034246..4b71ff8 100644
--- a/src/atm/main.py
+++ b/src/atm/main.py
@@ -556,7 +556,7 @@ def run_live(cfg, duration_s=None, capture_stub: bool = False) -> None:
     audit.log({"event": "started", "config": cfg.config_version, "canary": canary_status})
 
     start = time.monotonic()
-    heartbeat_due = start + cfg.heartbeat_min * 60
+    heartbeat_due = time.time() + cfg.heartbeat_min * 60
     levels_extractor = None
     last_saved_color: str | None = None
     first_accepted = True