TST: BUG49897 convert millis to micros so relativedelta can use them

Author: markopacak

pandas/_libs/tslibs/offsets.pyx

@@ -336,6 +336,11 @@ cdef _determine_offset(kwds):
         # This also handles "milliseconds" (plur): see GH 49897
         return timedelta(**kwds_no_nanos), False
 
+    # convert milliseconds to microseconds, so relativedelta can parse it
+    if "milliseconds" in kwds_no_nanos:
+        micro = kwds_no_nanos.pop("milliseconds") * 1000
+        kwds_no_nanos["microseconds"] = kwds_no_nanos.get("microseconds", 0) + micro
+
     if all(k in kwds_use_relativedelta for k in kwds_no_nanos):
         return relativedelta(**kwds_no_nanos), True
 

pandas/tests/tseries/offsets/test_offsets.py

@@ -742,10 +742,13 @@ def test_eq(self):
     @pytest.mark.parametrize(
         "offset_kwargs, expected_arg",
         [
+            ({"seconds": 1, "milliseconds": 1}, "2022-01-01 00:00:01.001"),
             ({"minutes": 1, "milliseconds": 1}, "2022-01-01 00:01:00.001"),
             ({"hours": 1, "milliseconds": 1}, "2022-01-01 01:00:00.001"),
             ({"days": 1, "milliseconds": 1}, "2022-01-02 00:00:00.001"),
             ({"weeks": 1, "milliseconds": 1}, "2022-01-08 00:00:00.001"),
+            ({"months": 1, "milliseconds": 1}, "2022-02-01 00:00:00.001"),
+            ({"years": 1, "milliseconds": 1}, "2023-01-01 00:00:00.001"),
         ],
     )
     def test_milliseconds_combination(self, offset_kwargs, expected_arg):