diff --git a/category-feed-json.njk b/category-feed-json.njk
index 7036984..c684207 100644
--- a/category-feed-json.njk
+++ b/category-feed-json.njk
@@ -56,7 +56,7 @@ permalink: "categories/{{ categoryFeed.slug }}/feed.json"
       "content_html": {{ post.content | htmlToAbsoluteUrls(absolutePostUrl) | jsonEncode | safe }},
       "content_text": {{ post.content | striptags | jsonEncode | safe }},
       "date_published": "{{ post.date | dateToRfc3339 }}",
-      "date_modified": "{{ (post.data.updated or post.date) | dateToRfc3339 }}"
+      "date_modified": "{{ (post.data.updated or post.date) | toDate | dateToRfc3339 }}"
       {%- if postImage and postImage != "" and (postImage | length) > 10 %},
       "image": "{{ postImage | url | absoluteUrl(site.url) }}"
       {%- endif %}
diff --git a/eleventy.config.js b/eleventy.config.js
index 4bbc3da..3bc9028 100644
--- a/eleventy.config.js
+++ b/eleventy.config.js
@@ -291,6 +291,13 @@ export default function (eleventyConfig) {
     return pluginRss.dateToRfc2822(date);
   });
 
+  // Coerce a value to a Date object (handles ISO strings from frontmatter like `updated:`)
+  eleventyConfig.addFilter("toDate", (value) => {
+    if (!value) return null;
+    if (value instanceof Date) return value;
+    return new Date(value);
+  });
+
   // Embed Everything - auto-embed YouTube, Vimeo, Bluesky, Mastodon, etc.
   // YouTube uses lite-yt-embed facade: shows thumbnail + play button,
   // only loads full iframe on click (~800 KiB savings).
diff --git a/feed-json.njk b/feed-json.njk
index 9ebdaad..9b13d3e 100644
--- a/feed-json.njk
+++ b/feed-json.njk
@@ -54,7 +54,7 @@ eleventyImport:
       "content_html": {{ (post.content | htmlToAbsoluteUrls(absolutePostUrl) + _jsonPixel) | jsonEncode | safe }},
       "content_text": {{ post.content | striptags | jsonEncode | safe }},
       "date_published": "{{ post.date | dateToRfc3339 }}",
-      "date_modified": "{{ (post.data.updated or post.date) | dateToRfc3339 }}"
+      "date_modified": "{{ (post.data.updated or post.date) | toDate | dateToRfc3339 }}"
       {%- if postImage and postImage != "" and (postImage | length) > 10 %},
       "image": "{{ postImage | url | absoluteUrl(site.url) }}"
       {%- endif %}