diff --git a/make_post_sell/views/feeds.py b/make_post_sell/views/feeds.py
index 0c310f5..f0a119d 100644
--- a/make_post_sell/views/feeds.py
+++ b/make_post_sell/views/feeds.py
@@ -14,12 +14,20 @@ from pyramid.view import view_config
 from ..models.product import Product
 
 
-def timestamp_to_iso8601(timestamp):
-    """Convert millisecond timestamp to ISO 8601 format."""
+def timestamp_to_sitemap_date(timestamp):
+    """Convert millisecond timestamp to YYYY-MM-DD format for sitemaps."""
     if timestamp:
-        dt = datetime.fromtimestamp(timestamp / 1000.0)
-        return dt.strftime("%Y-%m-%dT%H:%M:%S+00:00")
-    return datetime.utcnow().strftime("%Y-%m-%dT%H:%M:%S+00:00")
+        dt = datetime.utcfromtimestamp(timestamp / 1000.0)
+        return dt.strftime("%Y-%m-%d")
+    return datetime.utcnow().strftime("%Y-%m-%d")
+
+
+def timestamp_to_atom_datetime(timestamp):
+    """Convert millisecond timestamp to ISO 8601 format for Atom feeds."""
+    if timestamp:
+        dt = datetime.utcfromtimestamp(timestamp / 1000.0)
+        return dt.strftime("%Y-%m-%dT%H:%M:%SZ")
+    return datetime.utcnow().strftime("%Y-%m-%dT%H:%M:%SZ")
 
 
 def timestamp_to_rfc822(timestamp):
@@ -58,14 +66,14 @@ def build_sitemap_xml(request, shop, products, content):
     # Shop home page
     url = SubElement(urlset, "url")
     SubElement(url, "loc").text = shop.absolute_url(request)
-    SubElement(url, "lastmod").text = timestamp_to_iso8601(shop.updated_timestamp)
+    SubElement(url, "lastmod").text = timestamp_to_sitemap_date(shop.updated_timestamp)
     SubElement(url, "changefreq").text = "weekly"
     SubElement(url, "priority").text = "1.0"
 
     # Shop about page
     url = SubElement(urlset, "url")
     SubElement(url, "loc").text = shop.absolute_about_url(request)
-    SubElement(url, "lastmod").text = timestamp_to_iso8601(shop.updated_timestamp)
+    SubElement(url, "lastmod").text = timestamp_to_sitemap_date(shop.updated_timestamp)
     SubElement(url, "changefreq").text = "monthly"
     SubElement(url, "priority").text = "0.5"
 
@@ -73,7 +81,7 @@ def build_sitemap_xml(request, shop, products, content):
     if shop.terms_of_service_raw:
         url = SubElement(urlset, "url")
         SubElement(url, "loc").text = shop.absolute_terms_url(request)
-        SubElement(url, "lastmod").text = timestamp_to_iso8601(shop.updated_timestamp)
+        SubElement(url, "lastmod").text = timestamp_to_sitemap_date(shop.updated_timestamp)
         SubElement(url, "changefreq").text = "yearly"
         SubElement(url, "priority").text = "0.3"
 
@@ -81,7 +89,7 @@ def build_sitemap_xml(request, shop, products, content):
     if shop.privacy_policy_raw:
         url = SubElement(urlset, "url")
         SubElement(url, "loc").text = shop.absolute_privacy_policy_url(request)
-        SubElement(url, "lastmod").text = timestamp_to_iso8601(shop.updated_timestamp)
+        SubElement(url, "lastmod").text = timestamp_to_sitemap_date(shop.updated_timestamp)
         SubElement(url, "changefreq").text = "yearly"
         SubElement(url, "priority").text = "0.3"
 
@@ -89,7 +97,7 @@ def build_sitemap_xml(request, shop, products, content):
     for product in products:
         url = SubElement(urlset, "url")
         SubElement(url, "loc").text = product.absolute_url(request)
-        SubElement(url, "lastmod").text = timestamp_to_iso8601(product.updated_timestamp)
+        SubElement(url, "lastmod").text = timestamp_to_sitemap_date(product.updated_timestamp)
         SubElement(url, "changefreq").text = "weekly"
         SubElement(url, "priority").text = "0.8"
 
@@ -97,7 +105,7 @@ def build_sitemap_xml(request, shop, products, content):
     for item in content:
         url = SubElement(urlset, "url")
         SubElement(url, "loc").text = item.absolute_url(request)
-        SubElement(url, "lastmod").text = timestamp_to_iso8601(item.updated_timestamp)
+        SubElement(url, "lastmod").text = timestamp_to_sitemap_date(item.updated_timestamp)
         SubElement(url, "changefreq").text = "weekly"
         SubElement(url, "priority").text = "0.7"
 
@@ -150,7 +158,7 @@ def build_atom_xml(request, shop, products, content):
 
     SubElement(feed, "title").text = shop.name
     SubElement(feed, "id").text = shop.absolute_url(request)
-    SubElement(feed, "updated").text = timestamp_to_iso8601(shop.updated_timestamp)
+    SubElement(feed, "updated").text = timestamp_to_atom_datetime(shop.updated_timestamp)
 
     # Self link
     self_link = SubElement(feed, "link")
@@ -177,8 +185,8 @@ def build_atom_xml(request, shop, products, content):
         entry = SubElement(feed, "entry")
         SubElement(entry, "title").text = item_obj.title
         SubElement(entry, "id").text = item_obj.absolute_url(request)
-        SubElement(entry, "updated").text = timestamp_to_iso8601(item_obj.updated_timestamp)
-        SubElement(entry, "published").text = timestamp_to_iso8601(item_obj.created_timestamp)
+        SubElement(entry, "updated").text = timestamp_to_atom_datetime(item_obj.updated_timestamp)
+        SubElement(entry, "published").text = timestamp_to_atom_datetime(item_obj.created_timestamp)
 
         link = SubElement(entry, "link")
         link.set("href", item_obj.absolute_url(request))