BUG: Fix dt64[non_nano] + offset rounding

Author: Aniketsy

pandas/core/arrays/datetimes.py

@@ -826,10 +826,22 @@ def _add_offset(self, offset: BaseOffset) -> Self:
             res_unit = self.unit
             if hasattr(offset, "offset"):
                 offset_td = Timedelta(offset.offset)
-                offset_unit = offset_td.unit
+                ns = abs(int(offset_td.value))  # Timedelta.value is in nanoseconds
+                if ns == 0:
+                    eff_unit = self.unit
+                elif ns % 1_000_000_000 == 0:
+                    eff_unit = "s"
+                elif ns % 1_000_000 == 0:
+                    eff_unit = "ms"
+                elif ns % 1_000 == 0:
+                    eff_unit = "us"
+                else:
+                    eff_unit = "ns"
+
                 idx_self = units.index(self.unit)
-                idx_offset = units.index(offset_unit)
-                res_unit = units[min(idx_self, idx_offset)]
+                idx_eff = units.index(eff_unit)
+                res_unit = units[min(idx_self, idx_eff)]
+
             result = type(self)._simple_new(res_values, dtype=res_values.dtype)
             result = result.as_unit(res_unit)