diff --git a/JSON.prune.js b/JSON.prune.js
index 1137487..efb3aac 100644
--- a/JSON.prune.js
+++ b/JSON.prune.js
@@ -45,7 +45,9 @@
 		forEachProperty(Object.getPrototypeOf(obj), callback, excluded);
 	};
 
-	Object.defineProperty(Date.prototype, "toPrunedJSON", {value:Date.prototype.toJSON});
+	if (typeof Date.prototype.toPrunedJSON === "undefined") {
+		Object.defineProperty(Date.prototype, "toPrunedJSON", {value:Date.prototype.toJSON});
+	}
 
 	var	cx = /[\u0000\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,
 		escapable = /[\\\"\x00-\x1f\x7f-\x9f\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,