diff --git a/frontend/packages/agent/src/app.js b/frontend/packages/agent/src/app.js
index da1812d7..5790915b 100644
--- a/frontend/packages/agent/src/app.js
+++ b/frontend/packages/agent/src/app.js
@@ -894,10 +894,12 @@ window.marked = marked;
           buildAnswerForm(t.id),
         );
       } else if (t.kind === 'reminder') {
-        // due_at is an absolute unix-seconds value; show time-until-fire
-        // (negative when overdue, fmtAge handles 0/positive case here).
+        // due_at arrives as an ISO 8601 string (DateTime<Utc> on the wire).
+        // Parse it to unix seconds before arithmetic — doing `t.due_at - now`
+        // directly yields NaN because a string minus a number is NaN in JS.
         const now = Math.floor(Date.now() / 1000);
-        const dueIn = (t.due_at || 0) - now;
+        const dueAtSec = t.due_at ? Math.floor(new Date(t.due_at).getTime() / 1000) : 0;
+        const dueIn = dueAtSec - now;
         const dueLabel = dueIn >= 0 ? 'in ' + fmtAge(dueIn) : fmtAge(-dueIn) + ' overdue';
         li.append(
           el('span', { class: 'inbox-from' }, '⏰ reminder #' + t.id), ' ',