update whatsnew

Author: fbourgey

doc/source/whatsnew/v3.0.0.rst

@@ -715,6 +715,7 @@ Numeric
 - Bug in :meth:`DataFrame.quantile` where the column type was not preserved when ``numeric_only=True`` with a list-like ``q`` produced an empty result (:issue:`59035`)
 - Bug in :meth:`Series.dot` returning ``object`` dtype for :class:`ArrowDtype` and nullable-dtype data (:issue:`61375`)
 - Bug in ``np.matmul`` with :class:`Index` inputs raising a ``TypeError`` (:issue:`57079`)
+- Bug in :meth:`DataFrame.cov` where presence of ``NaT`` in columns could lead to incorrect results or errors (:issue:`53115`)
 
 Conversion
 ^^^^^^^^^^