diff --git a/linodecli/plugins/obj/__init__.py b/linodecli/plugins/obj/__init__.py
index 6952487f9..0142df92a 100644
--- a/linodecli/plugins/obj/__init__.py
+++ b/linodecli/plugins/obj/__init__.py
@@ -32,6 +32,7 @@
     ENV_ACCESS_KEY_NAME,
     ENV_SECRET_KEY_NAME,
     INCOMING_DATE_FORMAT,
+    MAX_SIGNED_URL_EXPIRY,
     MULTIPART_UPLOAD_CHUNK_SIZE_DEFAULT,
     NO_ACCESS_ERROR,
     NO_SCOPES_ERROR,
@@ -101,25 +102,45 @@ def generate_url(get_client, args, **kwargs):  # pylint: disable=unused-argument
         "expiry",
         metavar="EXPIRY",
         type=str,
-        help="When this link should expire.  Treated as an epoch "
-        "time if a number. If starts with a '+' treated as "
-        "an offset.",
+        help="When this link should expire. Treated as a Unix epoch "
+        "time in seconds if a number. If it starts with a '+', treated "
+        "as an offset in seconds from now. The link can be valid for "
+        f"at most {MAX_SIGNED_URL_EXPIRY} seconds (7 days).",
     )
 
-    # TODO:
-    # Add docs for date format and unit for 'expiry' parameter.
-
     parsed = parser.parse_args(args)
-    client = get_client()
 
     now = datetime.now()
 
-    if parsed.expiry.startswith("+"):
-        # this is an offset in seconds
-        offset = int(parsed.expiry[1:])
-    else:
-        expiry = int(parsed.expiry)
-        offset = expiry - ceil(now.timestamp())
+    try:
+        if parsed.expiry.startswith("+"):
+            # this is an offset in seconds
+            offset = int(parsed.expiry[1:])
+        else:
+            expiry = int(parsed.expiry)
+            offset = expiry - ceil(now.timestamp())
+    except ValueError:
+        print(
+            f"Invalid expiry {parsed.expiry!r}: expected an epoch time or "
+            "an offset like +3600, in seconds",
+            file=sys.stderr,
+        )
+        sys.exit(ExitCodes.ARGUMENT_ERROR)
+
+    if offset <= 0:
+        print("The expiry must be in the future", file=sys.stderr)
+        sys.exit(ExitCodes.ARGUMENT_ERROR)
+
+    if offset > MAX_SIGNED_URL_EXPIRY:
+        print(
+            f"The expiry is {offset} seconds from now, but a signed URL "
+            f"can be valid for at most {MAX_SIGNED_URL_EXPIRY} seconds "
+            "(7 days)",
+            file=sys.stderr,
+        )
+        sys.exit(ExitCodes.ARGUMENT_ERROR)
+
+    client = get_client()
 
     bucket = parsed.bucket
     key = parsed.file
diff --git a/linodecli/plugins/obj/config.py b/linodecli/plugins/obj/config.py
index 0e287f56c..5c835582e 100644
--- a/linodecli/plugins/obj/config.py
+++ b/linodecli/plugins/obj/config.py
@@ -28,6 +28,10 @@
 # for help commands
 PLUGIN_BASE = "linode-cli obj"
 
+# the longest a signed URL can stay valid, in seconds (7 days); a URL
+# signed with Signature Version 4 is refused once its expiry exceeds this
+MAX_SIGNED_URL_EXPIRY = 604800
+
 columns = shutil.get_terminal_size(fallback=(80, 24)).columns
 PROGRESS_BAR_WIDTH = columns - 20 if columns > 30 else columns
 
diff --git a/tests/unit/test_plugin_obj.py b/tests/unit/test_plugin_obj.py
index 608508099..a4f0e9c5b 100644
--- a/tests/unit/test_plugin_obj.py
+++ b/tests/unit/test_plugin_obj.py
@@ -1,11 +1,14 @@
 import time
-from unittest.mock import patch
+from unittest.mock import Mock, patch
 
+import pytest
 from pytest import CaptureFixture
 
 from linodecli import CLI, plugins
+from linodecli.exit_codes import ExitCodes
 from linodecli.plugins import obj
 from linodecli.plugins.obj import get_obj_args_parser, helpers, print_help
+from linodecli.plugins.obj.config import MAX_SIGNED_URL_EXPIRY
 
 
 def test_print_help(mock_cli: CLI, capsys: CaptureFixture):
@@ -336,3 +339,69 @@ def call_operation_side_effect(resource, action, *args, **kwargs):
         assert (
             not delete_calls
         ), "Cleanup should not be performed when key-cleanup-enabled is False"
+
+
+def test_generate_url_offset_within_limit(capsys: CaptureFixture):
+    client = Mock()
+    client.generate_presigned_url.return_value = "https://signed.example"
+
+    obj.generate_url(lambda: client, ["bucket", "key", "+3600"])
+
+    client.generate_presigned_url.assert_called_once_with(
+        ClientMethod="get_object",
+        ExpiresIn=3600,
+        Params={"Bucket": "bucket", "Key": "key"},
+    )
+    assert capsys.readouterr().out.strip() == "https://signed.example"
+
+
+def test_generate_url_offset_at_limit():
+    client = Mock()
+
+    obj.generate_url(
+        lambda: client, ["bucket", "key", f"+{MAX_SIGNED_URL_EXPIRY}"]
+    )
+
+    assert (
+        client.generate_presigned_url.call_args.kwargs["ExpiresIn"]
+        == MAX_SIGNED_URL_EXPIRY
+    )
+
+
+def test_generate_url_epoch_within_limit():
+    client = Mock()
+
+    obj.generate_url(
+        lambda: client, ["bucket", "key", str(int(time.time()) + 3600)]
+    )
+
+    assert (
+        3500
+        < client.generate_presigned_url.call_args.kwargs["ExpiresIn"]
+        <= 3600
+    )
+
+
+@pytest.mark.parametrize(
+    "expiry, message",
+    [
+        (f"+{MAX_SIGNED_URL_EXPIRY + 1}", "at most 604800 seconds"),
+        ("+31536000", "at most 604800 seconds"),
+        (str(int(time.time()) + 365 * 24 * 3600), "at most 604800 seconds"),
+        ("+0", "must be in the future"),
+        (str(int(time.time()) - 60), "must be in the future"),
+        ("+an-hour", "Invalid expiry"),
+        ("tomorrow", "Invalid expiry"),
+    ],
+)
+def test_generate_url_rejects_invalid_expiry(
+    capsys: CaptureFixture, expiry: str, message: str
+):
+    get_client = Mock()
+
+    with pytest.raises(SystemExit) as exc:
+        obj.generate_url(get_client, ["bucket", "key", expiry])
+
+    assert exc.value.code == ExitCodes.ARGUMENT_ERROR
+    assert message in capsys.readouterr().err
+    get_client.assert_not_called()