Fix conversion of datetimes in TransferData

The `deadline` field accepts a datetime but does not stringify it.
On `DeleteData`, the same field exists, with the same semantics, and is
passed to our internal `stringify()` helper. Apply the same here as the
fix so that the two remain consistent.
A new unit test verifies the results.

Note that this calls `str(deadline)`, which *does not* produce an
ISO 8601 date string. This was the behavior in the 3.x versions of the
SDK as well, which is part of why it is being kept rather than fixed at
this time.

Fixing it will require evaluating what the Transfer APIs accept.

Fixes #1371

Author: sirosen

changelog.d/20260227_094736_sirosen_bugfix_datetime_conversions.rst

@@ -0,0 +1,6 @@
+Fixed
+-----
+
+- Fixed a bug in ``globus_sdk.TransferData`` which failed to convert
+  ``deadline`` to a string when a ``datetime.datetime`` object is given.
+  (:pr:`NUMBER`)

src/globus_sdk/services/transfer/data/transfer_data.py

@@ -5,6 +5,7 @@
 import typing as t
 import uuid
 
+from globus_sdk._internal.remarshal import stringify
 from globus_sdk._missing import MISSING, MissingType
 from globus_sdk._payload import GlobusPayload
 
@@ -176,7 +177,7 @@ def __init__(
         self["destination_endpoint"] = destination_endpoint
         self["label"] = label
         self["submission_id"] = submission_id
-        self["deadline"] = deadline
+        self["deadline"] = stringify(deadline)
         self["source_local_user"] = source_local_user
         self["destination_local_user"] = destination_local_user
         self["verify_checksum"] = verify_checksum

tests/unit/helpers/test_transfer.py

@@ -1,3 +1,5 @@
+import datetime
+
 import pytest
 
 from globus_sdk import MISSING, DeleteData, TransferData
@@ -298,3 +300,17 @@ def test_add_filter_rule():
 )
 def test_ls_format_filter(filter, expected):
     assert _format_filter(filter) == expected
+
+
+@pytest.mark.parametrize(
+    "deadline_str",
+    [
+        pytest.param("2026-02-27 09:36:50.517299", id="naive_w_microseconds"),
+        pytest.param("2020-01-01 08:22:12+04:00", id="plus4"),
+    ],
+)
+def test_transfer_data_stringifies_deadline(deadline_str):
+    deadline_dt = datetime.datetime.fromisoformat(deadline_str)
+
+    tdata = TransferData(GO_EP1_ID, GO_EP2_ID, deadline=deadline_dt)
+    assert tdata["deadline"] == deadline_str