Updated the Datetimelike section in the file

Author: brandonmonge

doc/source/whatsnew/v3.0.0.rst

@@ -629,6 +629,7 @@ Datetimelike
 - Bug in :meth:`to_datetime` reports incorrect index in case of any failure scenario. (:issue:`58298`)
 - Bug in :meth:`to_datetime` wrongly converts when ``arg`` is a ``np.datetime64`` object with unit of ``ps``. (:issue:`60341`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
+- Fixed timezone and resolution preservation in :meth:`DatetimeIndex.union`. Previously, :meth:`DatetimeIndex.union` would sometimes convert timezone-aware indices with the same timezone but different units to UTC. (:issue:`60080`)
 
 Timedelta
 ^^^^^^^^^