Add changelog for new Timer creation

Also fix minor mypy issue.

changelog.d/20231027_012754_sirosen_timers_v2.rst

@@ -0,0 +1,19 @@
+Added
+~~~~~
+
+- Add support for the new Transfer Timer creation method, in the form of a
+  client method, ``TimerClient.create_timer``, and a payload builder type,
+  ``TransferTimer`` (:pr:`NUMBER`)
+
+  - ``create_timer`` only supports dict data and ``TransferTimer``, not the
+    previous ``TimerJob`` type
+
+  - Additional helper classes, ``RecurringTimerSchedule`` and
+    ``OneceTimerSchedule``, are provided to help build the ``TransferTimer``
+    payload
+
+Deprecated
+~~~~~~~~~~
+
+- Creation of timers to run transfers using ``TimerJob`` is now
+  deprecated (:pr:`NUMBER`)

src/globus_sdk/services/timer/data.py

@@ -350,7 +350,7 @@ def from_transfer_data(
         )
 
 
-def _format_date(date: str | dt.datetime | MISSING) -> str | MISSING:
+def _format_date(date: str | dt.datetime | MissingType) -> str | MissingType:
     if isinstance(date, dt.datetime):
         return date.astimezone(dt.timezone.utc).replace(microsecond=0).isoformat()
     else: