diff --git a/assets/layers/defibrillator/defibrillator.json b/assets/layers/defibrillator/defibrillator.json
index c5b1e35d73..ba55143d30 100644
--- a/assets/layers/defibrillator/defibrillator.json
+++ b/assets/layers/defibrillator/defibrillator.json
@@ -23,7 +23,7 @@
     "osmTags": "emergency=defibrillator"
   },
   "calculatedTags": [
-    "_days_since_last_survey=Math.floor(new Date() - new Date(feat.properties['survey:date'])/(1000*60*60*24))",
+    "_days_since_last_survey=Math.floor((new Date() - new Date(feat.properties['survey:date']))/(1000*60*60*24))",
     "_recently_surveyed=Number(feat.properties._days_since_last_survey) <= 90"
   ],
   "minzoom": 12,