Index: src/middleware/render.js
===================================================================
--- src/middleware/render.js	(revision a7bcf669de39441a2bbc7c41545fc9e17db4b233)
+++ src/middleware/render.js	(revision 02021042f8346f4cca62228062484c41e0df8bcd)
@@ -97,4 +97,8 @@
 const parseStamp = (v) => {
   if (!v) return null;
+  if (typeof v === 'number') {   // epoch ms (e.g. an availability endTime)
+    const d = new Date(v);
+    return Number.isNaN(d.getTime()) ? null : d;
+  }
   const s = String(v);
   const d = /^\d{4}-\d{2}-\d{2}[ T]\d{2}:\d{2}(:\d{2})?$/.test(s)