BUG: Fix dt64[non_nano] + offset rounding

Author: Aniketsy

pandas/core/arrays/datetimes.py

@@ -802,6 +802,8 @@ def _add_offset(self, offset: BaseOffset) -> Self:
         else:
             values = self
 
+        units = ["ns", "us", "ms", "s"]
+
         try:
             res_values = offset._apply_array(values._ndarray)
             if res_values.dtype.kind == "i":
@@ -814,45 +816,27 @@ def _add_offset(self, offset: BaseOffset) -> Self:
                     PerformanceWarning,
                     stacklevel=find_stack_level(),
                 )
-
             # Handle non-vectorized DateOffsets with both calendar and
             # timedelta parts, preserving time resolution.
-            if getattr(offset, "offset", None):
-                try:
-                    kwds = offset.kwds.copy()
-                    if "offset" in kwds:
-                        del kwds["offset"]
-                        base_offset = type(offset)(**kwds)
-                        result = self._add_offset(base_offset) + offset.offset
-
-                        offset_td = Timedelta(offset.offset)
-                        offset_unit = offset_td.unit
-                        if offset_unit == "ns":
-                            res_str = offset_td.resolution_string
-                            if res_str in ["ms", "us", "s"]:
-                                offset_unit = res_str
-
-                        units = ["ns", "us", "ms", "s"]
-                        if self.unit in units and offset_unit in units:
-                            idx_self = units.index(self.unit)
-                            idx_offset = units.index(offset_unit)
-                            res_unit = units[min(idx_self, idx_offset)]
-                            return result.as_unit(res_unit)
-
-                        return result
-                except Exception:
-                    pass
-
-            res_values = self.astype("O") + offset
-            result = type(self)._from_sequence(res_values, dtype=self.dtype)
+            values_ns = values.as_unit("ns") if values.unit != "ns" else values
+            res_values = np.array([ts + offset for ts in values_ns], dtype=object)
+
+            res_unit = self.unit
+
+            off = getattr(offset, "offset", None)
+            if off is not None:
+                offset_td = Timedelta(off)
+                if offset_td.value != 0:
+                    offset_unit = offset_td.resolution_string
+                    if self.unit in units and offset_unit in units:
+                        idx_self = units.index(self.unit)
+                        idx_offset = units.index(offset_unit)
+                        res_unit = units[min(idx_self, idx_offset)]
+
+            dtype = np.dtype(f"datetime64[{res_unit}]")
+            result = type(self)._from_sequence(res_values, dtype=dtype)
 
         else:
-            units = [
-                "ns",
-                "us",
-                "ms",
-                "s",
-            ]
             res_unit = self.unit
             if type(offset) is DateOffset:
                 if "nanoseconds" in offset.kwds:
@@ -869,12 +853,12 @@ def _add_offset(self, offset: BaseOffset) -> Self:
             result = type(self)._simple_new(res_values, dtype=res_values.dtype)
             result = result.as_unit(res_unit)
 
-            if offset.normalize:
-                result = result.normalize()
-                result._freq = None
+        if offset.normalize:
+            result = result.normalize()
+            result._freq = None
 
-            if self.tz is not None:
-                result = result.tz_localize(self.tz)
+        if self.tz is not None:
+            result = result.tz_localize(self.tz)
 
         return result