BUG: DateOffset addition preserve non-nano (#47334)

Author: jbrockmendel

pandas/_libs/tslibs/offsets.pyx

@@ -70,6 +70,7 @@ from pandas._libs.tslibs.np_datetime cimport (
 
 from .dtypes cimport PeriodDtypeCode
 from .timedeltas cimport (
+    _Timedelta,
     delta_to_nanoseconds,
     is_any_td_scalar,
 )
@@ -1140,7 +1141,9 @@ cdef class RelativeDeltaOffset(BaseOffset):
 
             weeks = kwds.get("weeks", 0) * self.n
             if weeks:
-                dt64other = dt64other + Timedelta(days=7 * weeks)
+                delta = Timedelta(days=7 * weeks)
+                td = (<_Timedelta>delta)._as_reso(reso)
+                dt64other = dt64other + td
 
             timedelta_kwds = {
                 k: v
@@ -1149,13 +1152,14 @@ cdef class RelativeDeltaOffset(BaseOffset):
             }
             if timedelta_kwds:
                 delta = Timedelta(**timedelta_kwds)
-                dt64other = dt64other + (self.n * delta)
-                # FIXME: fails to preserve non-nano
+                td = (<_Timedelta>delta)._as_reso(reso)
+                dt64other = dt64other + (self.n * td)
             return dt64other
         elif not self._use_relativedelta and hasattr(self, "_offset"):
             # timedelta
-            # FIXME: fails to preserve non-nano
-            return dt64other + Timedelta(self._offset * self.n)
+            delta = Timedelta(self._offset * self.n)
+            td = (<_Timedelta>delta)._as_reso(reso)
+            return dt64other + td
         else:
             # relativedelta with other keywords
             kwd = set(kwds) - relativedelta_fast

pandas/tests/tseries/offsets/test_offsets.py

@@ -556,10 +556,6 @@ def test_add_dt64_ndarray_non_nano(self, offset_types, unit, request):
         # check that the result with non-nano matches nano
         off = self._get_offset(offset_types)
 
-        if type(off) is DateOffset:
-            mark = pytest.mark.xfail(reason="non-nano not implemented")
-            request.node.add_marker(mark)
-
         dti = date_range("2016-01-01", periods=35, freq="D")
 
         arr = dti._data._ndarray.astype(f"M8[{unit}]")