diff --git a/news/93.feature b/news/93.feature
new file mode 100644
index 0000000..fe0847c
--- /dev/null
+++ b/news/93.feature
@@ -0,0 +1,4 @@
+Update the resourceRegistries ETag to use the config registry modification time.
+This time is set since Plone 6.0.4.
+Fixes `issue 93 <https://github.com/plone/plone.app.caching/issues/93>`_.
+[maurits]
diff --git a/plone/app/caching/operations/etags.py b/plone/app/caching/operations/etags.py
index 8187172..20f1783 100644
--- a/plone/app/caching/operations/etags.py
+++ b/plone/app/caching/operations/etags.py
@@ -4,11 +4,10 @@
 from plone.app.caching.operations.utils import getContext
 from plone.app.caching.operations.utils import getLastModifiedAnnotation
 from plone.base.utils import safe_hasattr
+from plone.registry.interfaces import IRegistry
 from Products.CMFCore.interfaces import ICatalogTool
 from Products.CMFCore.interfaces import IMembershipTool
 from Products.CMFCore.utils import getToolByName
-from Products.CMFPlone.resources.utils import get_override_directory
-from Products.CMFPlone.resources.utils import PRODUCTION_RESOURCE_DIRECTORY
 from zope.component import adapter
 from zope.component import queryMultiAdapter
 from zope.component import queryUtility
@@ -19,6 +18,13 @@
 import time
 
 
+try:
+    # available since Plone 6.0.4
+    from Products.CMFPlone.resources.browser.resource import _RESOURCE_REGISTRY_MTIME
+except ImportError:
+    _RESOURCE_REGISTRY_MTIME = None
+
+
 @implementer(IETagValue)
 @adapter(Interface, Interface)
 class UserID:
@@ -239,22 +245,15 @@ def __init__(self, published, request):
         self.request = request
 
     def __call__(self):
-        context = getContext(self.published)
-        container = get_override_directory(context)
-        if PRODUCTION_RESOURCE_DIRECTORY not in container:
+        if _RESOURCE_REGISTRY_MTIME is None:
             return ""
-        production_folder = container[PRODUCTION_RESOURCE_DIRECTORY]
-        filename = "timestamp.txt"
-        if filename not in production_folder:
+        registry = queryUtility(IRegistry)
+        if registry is None:
             return ""
-        timestamp = production_folder.readFile(filename)
-        if not timestamp:
+        mtime = getattr(registry, _RESOURCE_REGISTRY_MTIME, None)
+        if mtime is None:
             return ""
-        # timestamp is in bytes, and we must return a string.
-        # On Python 2 this is the same, but not on Python 3.
-        if not isinstance(timestamp, str):
-            timestamp = timestamp.decode("utf-8")
-        return timestamp
+        return str(mtime)
 
 
 @implementer(IETagValue)
diff --git a/plone/app/caching/tests/test_profile_with_caching_proxy.py b/plone/app/caching/tests/test_profile_with_caching_proxy.py
index 8df999a..cdb62c9 100644
--- a/plone/app/caching/tests/test_profile_with_caching_proxy.py
+++ b/plone/app/caching/tests/test_profile_with_caching_proxy.py
@@ -73,12 +73,12 @@ def test_composite_viewsxx(self):
         # Can we just call that test from this context?
 
         catalog = self.portal["portal_catalog"]
-        skins_tool = self.portal["portal_skins"]
+        default_skin = self.portal["portal_skins"].default_skin
 
         # Add folder content
         setRoles(self.portal, TEST_USER_ID, ("Manager",))
         self.portal.invokeFactory("Folder", "f1")
-        self.portal["f1"].title = "one"
+        self.portal["f1"].title = "Folder one"
         self.portal["f1"].description = "Folder one description"
         self.portal["f1"].reindexObject()
 
@@ -105,9 +105,16 @@ def test_composite_viewsxx(self):
         # - turn on gzip?
         # - set skin?  Maybe
         # - leave status unlocked
-        #
+        # - set the mod date on the resource registries?  Probably.
         transaction.commit()
 
+        # Since Plone 6.0.4 we have a modification date on the registry.
+        from Products.CMFPlone.resources.browser.resource import (
+            _RESOURCE_REGISTRY_MTIME,
+        )
+
+        mtime = str(getattr(self.registry, _RESOURCE_REGISTRY_MTIME))
+
         # Request the authenticated folder
         now = stable_now()
         browser = Browser(self.app)
@@ -124,10 +131,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"|test_user_1_|{catalog.getCounter()}|en|{skins_tool.default_skin}|0|0|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|0|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
         # Set the copy/cut cookie and then request the folder view again
@@ -141,10 +146,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"|test_user_1_|{catalog.getCounter()}|en|{skins_tool.default_skin}|0|1|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|1|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
 
         # Request the authenticated page
         now = stable_now()
@@ -163,10 +166,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"|test_user_1_|{catalog.getCounter()}|en|{skins_tool.default_skin}|0|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
         # Request the authenticated page again -- to test RAM cache.
@@ -205,7 +206,6 @@ def test_composite_viewsxx(self):
         # Request the anonymous folder
         now = stable_now()
         browser = Browser(self.app)
-        browser.handleErrors = False
         browser.open(self.portal["f1"].absolute_url())
         self.assertEqual("plone.content.folderView", browser.headers["X-Cache-Rule"])
         self.assertEqual(
@@ -215,10 +215,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"||{catalog.getCounter()}|en|{skins_tool.default_skin}|0|0|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|0|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
         # Request the anonymous page
@@ -234,10 +232,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"||{catalog.getCounter()}|en|{skins_tool.default_skin}|0|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
         # Request the anonymous page again -- to test RAM cache.
@@ -257,10 +253,8 @@ def test_composite_viewsxx(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        self.assertEqual(
-            f'"||{catalog.getCounter()}|en|{skins_tool.default_skin}|0|"',
-            normalize_etag(browser.headers["ETag"]),
-        )
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
+        self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
         # Request the anonymous page again -- with an INM header to test 304.
diff --git a/plone/app/caching/tests/test_profile_without_caching_proxy.py b/plone/app/caching/tests/test_profile_without_caching_proxy.py
index 07a0585..3d9076a 100644
--- a/plone/app/caching/tests/test_profile_without_caching_proxy.py
+++ b/plone/app/caching/tests/test_profile_without_caching_proxy.py
@@ -92,6 +92,13 @@ def test_composite_views(self):
         # - set the mod date on the resource registries?  Probably.
         transaction.commit()
 
+        # Since Plone 6.0.4 we have a modification date on the registry.
+        from Products.CMFPlone.resources.browser.resource import (
+            _RESOURCE_REGISTRY_MTIME,
+        )
+
+        mtime = str(getattr(self.registry, _RESOURCE_REGISTRY_MTIME))
+
         # Request the authenticated folder
         now = stable_now()
         browser = Browser(self.app)
@@ -108,7 +115,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|0|"'
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|0|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
@@ -123,7 +130,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|1|"'
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|1|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
 
         # Request the authenticated page
@@ -143,7 +150,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|"'
+        tag = f'"|test_user_1_|{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
@@ -192,7 +199,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|0|"'
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|0|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
@@ -209,7 +216,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|"'
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))
 
@@ -230,7 +237,7 @@ def test_composite_views(self):
         self.assertEqual(
             "max-age=0, must-revalidate, private", browser.headers["Cache-Control"]
         )
-        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|"'
+        tag = f'"||{catalog.getCounter()}|en|{default_skin}|0|{mtime}"'
         self.assertEqual(tag, normalize_etag(browser.headers["ETag"]))
         self.assertGreater(now, dateutil.parser.parse(browser.headers["Expires"]))