ERR: Raise NotImplimentedError for EWMA with times and adjust=False (#40314)

Author: mroeschke

doc/source/whatsnew/v1.3.0.rst

@@ -570,6 +570,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrameGroupBy.sample` where column selection was not applied to sample result (:issue:`39928`)
 - Bug in :class:`core.window.ewm.ExponentialMovingWindow` when calling ``__getitem__`` would incorrectly raise a ``ValueError`` when providing ``times`` (:issue:`40164`)
 - Bug in :class:`core.window.ewm.ExponentialMovingWindow` when calling ``__getitem__`` would not retain ``com``, ``span``, ``alpha`` or ``halflife`` attributes  (:issue:`40164`)
+- :class:`core.window.ewm.ExponentialMovingWindow` now raises a ``NotImplementedError`` when specifying ``times`` with ``adjust=False`` due to an incorrect calculation (:issue:`40098`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/window/ewm.py

@@ -255,6 +255,8 @@ def __init__(
         self.ignore_na = ignore_na
         self.times = times
         if self.times is not None:
+            if not self.adjust:
+                raise NotImplementedError("times is not supported with adjust=False.")
             if isinstance(self.times, str):
                 self.times = self._selected_obj[self.times]
             if not is_datetime64_ns_dtype(self.times):

pandas/tests/window/test_ewm.py

@@ -171,3 +171,13 @@ def test_ewm_vol_deprecated():
         result = ser.ewm(com=0.1).vol()
     expected = ser.ewm(com=0.1).std()
     tm.assert_series_equal(result, expected)
+
+
+def test_ewma_times_adjust_false_raises():
+    # GH 40098
+    with pytest.raises(
+        NotImplementedError, match="times is not supported with adjust=False."
+    ):
+        Series(range(1)).ewm(
+            0.1, adjust=False, times=date_range("2000", freq="D", periods=1)
+        )