BUG: pd.DateOffset handle milliseconds

doc/source/whatsnew/v2.0.0.rst

@@ -638,6 +638,7 @@ Datetimelike
 - Bug in subtracting a ``datetime`` scalar from :class:`DatetimeIndex` failing to retain the original ``freq`` attribute (:issue:`48818`)
 - Bug in ``pandas.tseries.holiday.Holiday`` where a half-open date interval causes inconsistent return types from :meth:`USFederalHolidayCalendar.holidays` (:issue:`49075`)
 - Bug in rendering :class:`DatetimeIndex` and :class:`Series` and :class:`DataFrame` with timezone-aware dtypes with ``dateutil`` or ``zoneinfo`` timezones near daylight-savings transitions (:issue:`49684`)
+- Bug in :class:`DateOffset` when constructing with milliseconds in combination with another argument (:issue:`49897`)
 -
 
 Timedelta

pandas/_libs/tslibs/offsets.pyx

@@ -314,6 +314,9 @@ cdef _determine_offset(kwds):
 
     use_relativedelta = False
     if len(kwds_no_nanos) > 0:
+        if "milliseconds" in kwds_no_nanos:
+            return timedelta(**kwds_no_nanos), use_relativedelta
+
         if any(k in _kwds_use_relativedelta for k in kwds_no_nanos):
             if "millisecond" in kwds_no_nanos:
                 raise NotImplementedError(
@@ -1163,7 +1166,6 @@ cdef class RelativeDeltaOffset(BaseOffset):
 
     def __init__(self, n=1, normalize=False, **kwds):
         BaseOffset.__init__(self, n, normalize)
-
         off, use_rd = _determine_offset(kwds)
         object.__setattr__(self, "_offset", off)
         object.__setattr__(self, "_use_relativedelta", use_rd)

pandas/tests/tseries/offsets/test_offsets.py

@@ -739,6 +739,14 @@ def test_eq(self):
 
         assert DateOffset(milliseconds=3) != DateOffset(milliseconds=7)
 
+    def test_milliseconds_combination(self):
+        """
+        The combination of 'milliseconds' (plural) and another parameter
+        should not raise an error.
+        See https://github.com/pytest-dev/pytest/issues/10525
+        """
+        DateOffset(days=1, milliseconds=1)
+
 
 class TestOffsetNames:
     def test_get_offset_name(self):