diff --git a/docs/backends.rst b/docs/backends.rst
index ef9b63f..b9b8c0a 100644
--- a/docs/backends.rst
+++ b/docs/backends.rst
@@ -23,6 +23,8 @@ Use storage on a local POSIX filesystem:
 - Namespaces: directories
 - Values: in key-named files
 - atime: supported (require fs atime support)
+- mtime: supported (stamped by the filesystem's clock - for a network filesystem
+  that is usually the file server's clock)
 - Quota: tracks backend storage size and rejects ``store`` if quota is exceeded.
 
   The current usage is persisted to a hidden file in the storage directory.
@@ -121,6 +123,7 @@ Use storage on an SFTP server:
 - Values: in key-named files
 - store: a ``memoryview`` value is copied into a ``bytes`` object first, because
   paramiko does not accept a ``memoryview``.
+- atime / mtime: supported (stamped by the SFTP server's filesystem)
 - hash: runs the hexdigest computation server-side (if server supports check-file).
   "blake3" is not part of the check-file extension, so it is always computed client-side.
 
@@ -134,6 +137,8 @@ Use storage on any of the many cloud providers `rclone <https://rclone.org/>`_ s
 - The implementation primarily depends on the specific remote.
 - The rclone binary path can be set via the environment variable ``RCLONE_BINARY`` (default: "rclone").
 - Debugging of HTTP requests/responses can be enabled by setting ``BORGSTORE_RCLONE_DEBUG=1``.
+- atime / mtime: not supported (always 0). rclone's ModTime is the client-side mtime
+  preserved across upload, not a timestamp stamped by the storage side.
 
 
 s3
@@ -163,6 +168,8 @@ Use storage on an S3-compliant cloud service:
 - Values: in key-named files
 - store: a ``memoryview`` value is copied into a ``bytes`` object first, because
   boto3 does not accept a ``memoryview``.
+- atime: not supported (always 0).
+- mtime: supported (``LastModified``, stamped by the S3 service).
 
 
 REST (http/https)
@@ -180,4 +187,5 @@ Use a storage backend running inside a BorgStore REST server process:
 - hash: runs the hexdigest computation server-side. Using algorithm "blake3" requires
   the optional ``blake3`` package to be installed **on the server**.
 - defrag: runs the defragmentation helper server-side.
-- atime: supported (if backend used by server supports it).
+- atime / mtime: supported (if backend used by server supports it). mtime is stamped
+  by the server side (the store operation executes there).
diff --git a/docs/changes.rst b/docs/changes.rst
index 24c65b8..3195214 100644
--- a/docs/changes.rst
+++ b/docs/changes.rst
@@ -1,6 +1,17 @@
 Changelog
 =========
 
+Version 0.6.1 (not released yet)
+--------------------------------
+
+New features:
+
+- ItemInfo: new ``mtime`` field - the last modification timestamp stamped by the
+  *storage side's* clock (0 if unknown). Implemented for posixfs, sftp, s3 and
+  rest; rclone reports 0 because its ModTime is a client-supplied timestamp.
+  sftp additionally reports atime now.
+
+
 Version 0.6.0 (2026-08-02)
 --------------------------
 
diff --git a/src/borgstore/backends/_base.py b/src/borgstore/backends/_base.py
index 990720a..da9f69f 100644
--- a/src/borgstore/backends/_base.py
+++ b/src/borgstore/backends/_base.py
@@ -11,8 +11,11 @@
 from ..constants import MAX_NAME_LENGTH, TMP_SUFFIX, HID_SUFFIX
 from ..utils import hashing
 
-# atime is the last read access UNIX timestamp [s] or 0 if not implemented
-ItemInfo = namedtuple("ItemInfo", "name exists size directory atime", defaults=(0,))
+# atime is the last read access UNIX timestamp [s] or 0 if not implemented.
+# mtime is the last modification UNIX timestamp [s] or 0 if not implemented - it must be
+# stamped by the *storage side's* clock; backends that would only echo a client-supplied
+# timestamp (e.g. rclone) must report 0 (unknown) instead.
+ItemInfo = namedtuple("ItemInfo", "name exists size directory atime mtime", defaults=(0, 0))
 
 # type of a value given to store: a memoryview is accepted in addition to bytes,
 # so callers can avoid copying (e.g. give a slice of a big buffer they already have).
diff --git a/src/borgstore/backends/posixfs.py b/src/borgstore/backends/posixfs.py
index 7d4d3e2..d8c48c5 100644
--- a/src/borgstore/backends/posixfs.py
+++ b/src/borgstore/backends/posixfs.py
@@ -205,7 +205,9 @@ def info(self, name):
             return ItemInfo(name=path.name, exists=False, directory=False, size=0)
         else:
             is_dir = stat.S_ISDIR(st.st_mode)
-            return ItemInfo(name=path.name, exists=True, directory=is_dir, size=st.st_size, atime=st.st_atime)
+            return ItemInfo(
+                name=path.name, exists=True, directory=is_dir, size=st.st_size, atime=st.st_atime, mtime=st.st_mtime
+            )
 
     def load(self, name, *, size=None, offset=0):
         if not self.opened:
@@ -355,7 +357,14 @@ def list(self, name):
                         pass
                     else:
                         is_dir = stat.S_ISDIR(st.st_mode)
-                        yield ItemInfo(name=p.name, exists=True, size=st.st_size, directory=is_dir, atime=st.st_atime)
+                        yield ItemInfo(
+                            name=p.name,
+                            exists=True,
+                            size=st.st_size,
+                            directory=is_dir,
+                            atime=st.st_atime,
+                            mtime=st.st_mtime,
+                        )
 
     def quota(self) -> dict:
         """Return quota information: limit and usage in bytes. -1 means not set / not tracked."""
diff --git a/src/borgstore/backends/rclone.py b/src/borgstore/backends/rclone.py
index 66060e2..461b1ef 100644
--- a/src/borgstore/backends/rclone.py
+++ b/src/borgstore/backends/rclone.py
@@ -251,6 +251,10 @@ def _to_item_info(self, remote, item):
         name = item["Name"]
         size = item["Size"]
         directory = item["IsDir"]
+        # no atime/mtime: rclone's ModTime is the *client-side* mtime preserved across upload
+        # (not stamped by the storage side), so it must not be reported as ItemInfo.mtime;
+        # also, fetching it would need "noModTime": False, which on several remotes costs an
+        # extra metadata read per object during list().
         return ItemInfo(name=name, exists=True, size=size, directory=directory)
 
     def info(self, name) -> ItemInfo:
diff --git a/src/borgstore/backends/rest.py b/src/borgstore/backends/rest.py
index c5970f0..6d4a2fa 100644
--- a/src/borgstore/backends/rest.py
+++ b/src/borgstore/backends/rest.py
@@ -526,8 +526,9 @@ def info(self, name: str) -> ItemInfo:
         exists = response.status_code == HTTP.OK
         is_dir = response.headers.get("X-BorgStore-Is-Directory") == "true"
         atime = float(response.headers.get("X-BorgStore-Atime", 0))
+        mtime = float(response.headers.get("X-BorgStore-Mtime", 0))  # 0: old server without mtime support
         size = int(response.headers.get("Content-Length", 0)) if exists else 0
-        return ItemInfo(name=name, exists=exists, size=size, directory=is_dir, atime=atime)
+        return ItemInfo(name=name, exists=exists, size=size, directory=is_dir, atime=atime, mtime=mtime)
 
     @with_reconnect
     def load(self, name: str, *, size=None, offset=0) -> bytes:
@@ -633,4 +634,5 @@ def list(self, name: str) -> Iterator[ItemInfo]:
                 size=entry["size"],
                 directory=entry.get("directory", False),
                 atime=entry.get("atime", 0),
+                mtime=entry.get("mtime", 0),  # 0: old server without mtime support
             )
diff --git a/src/borgstore/backends/s3.py b/src/borgstore/backends/s3.py
index 438e12b..eabb6d6 100644
--- a/src/borgstore/backends/s3.py
+++ b/src/borgstore/backends/s3.py
@@ -273,7 +273,13 @@ def list(self, name):
                         pass  # that file is likely not from us or is still uploading
                     else:
                         start_after = obj["Key"]
-                        yield ItemInfo(name=obj_name, exists=True, size=obj["Size"], directory=False)
+                        yield ItemInfo(
+                            name=obj_name,
+                            exists=True,
+                            size=obj["Size"],
+                            directory=False,
+                            mtime=obj["LastModified"].timestamp(),
+                        )
                 for prefix in objects.get("CommonPrefixes", []):
                     dir_name = prefix["Prefix"][len(base_prefix) : -1]  # Remove base_path prefix and trailing slash
                     yield ItemInfo(name=dir_name, exists=True, size=0, directory=True)
@@ -303,7 +309,13 @@ def info(self, name):
         key = self.base_path + name
         try:
             obj = self.s3.head_object(Bucket=self.bucket, Key=key)
-            return ItemInfo(name=name, exists=True, directory=False, size=obj["ContentLength"])
+            return ItemInfo(
+                name=name,
+                exists=True,
+                directory=False,
+                size=obj["ContentLength"],
+                mtime=obj["LastModified"].timestamp(),
+            )
         except self.s3.exceptions.ClientError as e:
             if e.response["Error"]["Code"] == "404":
                 try:
diff --git a/src/borgstore/backends/sftp.py b/src/borgstore/backends/sftp.py
index a14aee2..9d46cb0 100644
--- a/src/borgstore/backends/sftp.py
+++ b/src/borgstore/backends/sftp.py
@@ -412,7 +412,9 @@ def info(self, name):
             return ItemInfo(name=name, exists=False, directory=False, size=0)
         else:
             is_dir = stat.S_ISDIR(st.st_mode)
-            return ItemInfo(name=name, exists=True, directory=is_dir, size=st.st_size)
+            return ItemInfo(
+                name=name, exists=True, directory=is_dir, size=st.st_size, atime=st.st_atime, mtime=st.st_mtime
+            )
 
     @with_reconnect
     def load(self, name, *, size=None, offset=0):
@@ -546,4 +548,11 @@ def list(self, name):
                 pass  # that file is likely not from us or is still uploading
             else:
                 is_dir = stat.S_ISDIR(info.st_mode)
-                yield ItemInfo(name=info.filename, exists=True, size=info.st_size, directory=is_dir)
+                yield ItemInfo(
+                    name=info.filename,
+                    exists=True,
+                    size=info.st_size,
+                    directory=is_dir,
+                    atime=info.st_atime,
+                    mtime=info.st_mtime,
+                )
diff --git a/src/borgstore/server/rest.py b/src/borgstore/server/rest.py
index fdc7a9f..1b456b9 100644
--- a/src/borgstore/server/rest.py
+++ b/src/borgstore/server/rest.py
@@ -310,6 +310,7 @@ def do_HEAD(self):
                     "Content-Length": str(info.size),
                     "X-BorgStore-Is-Directory": "true" if info.directory else "false",
                     "X-BorgStore-Atime": str(info.atime),
+                    "X-BorgStore-Mtime": str(info.mtime),
                 },
             )
         except Exception as e:
@@ -324,7 +325,13 @@ def do_GET(self):
                 # [{"name": "...", "size": ...}, ...]
                 with self.server.backend:
                     items = (
-                        {"name": item.name, "size": item.size, "directory": item.directory, "atime": item.atime}
+                        {
+                            "name": item.name,
+                            "size": item.size,
+                            "directory": item.directory,
+                            "atime": item.atime,
+                            "mtime": item.mtime,
+                        }
                         for item in self.server.backend.list(self.name)
                     )
                     json_data = json.dumps(list(items), indent=2)
diff --git a/tests/test_backends.py b/tests/test_backends.py
index 9faefe6..4f90389 100644
--- a/tests/test_backends.py
+++ b/tests/test_backends.py
@@ -540,6 +540,9 @@ def test_list(tested_backends, request):
         assert matching_k1[0].exists and not matching_k1[0].directory and matching_k1[0].size == len(v1)
         assert matching_k0[0].atime >= 0
         assert matching_k1[0].atime >= 0
+        # mtime is 0 for backends that can not provide a storage-side timestamp (e.g. rclone)
+        assert matching_k0[0].mtime >= 0
+        assert matching_k1[0].mtime >= 0
         # for "dir", we do not know what size the backend has returned.
         # that is rather OS / fs / backend specific.
         matching_items = [item for item in items if item.name == "dir"]
diff --git a/tests/test_server_rest.py b/tests/test_server_rest.py
index 0bc90b1..5bef820 100644
--- a/tests/test_server_rest.py
+++ b/tests/test_server_rest.py
@@ -699,12 +699,14 @@ def do_request(method, path, body=b""):
         assert status == 200
         items = json.loads(body.decode("utf-8"))
         assert any(item["name"] == "item1" and item.get("atime", 0) > 0 for item in items)
+        assert any(item["name"] == "item1" and item.get("mtime", 0) > 0 for item in items)
 
         # 4. Info (HEAD)
         status, body, headers = do_request("HEAD", "/item1")
         assert status == 200
         assert body == b""
         assert float(headers.get("X-BorgStore-Atime", 0)) > 0
+        assert float(headers.get("X-BorgStore-Mtime", 0)) > 0
 
         # 5. Info for nonexistent (HEAD)
         status, body, headers = do_request("HEAD", "/nonexistent")
@@ -740,12 +742,14 @@ def test_rest_url(tmp_path):
         assert info.exists
         assert info.size == len(item_data)
         assert info.atime > 0
+        assert info.mtime > 0
 
         # Test listing
         items = list(store.list(""))
         assert len(items) == 1
         assert items[0].name == item_name
         assert items[0].atime > 0
+        assert items[0].mtime > 0
 
         # Test nonexistent item
         # This also used to hang if it returned a 404 with a body.
diff --git a/tests/test_store.py b/tests/test_store.py
index b82980a..9310299 100644
--- a/tests/test_store.py
+++ b/tests/test_store.py
@@ -4,6 +4,8 @@
 
 import array
 import hashlib
+import time
+
 import pytest
 
 from . import key, list_store_names, list_store_names_sorted
@@ -90,6 +92,9 @@ def test_basics(posixfs_store_created):
         assert items[0].size == len(v0)
         assert not items[0].directory
         assert items[0].atime >= 0
+        # posixfs: mtime is the (local) filesystem clock at store() time
+        assert abs(items[0].mtime - time.time()) < 120
+        assert abs(store.info(nsk0).mtime - time.time()) < 120
 
         store.delete(nsk0)