BUG: Timedelta.round() raises ZeroDivisionError when internal unit is 's' and target frequency is sub-second

Author: kjmin622

doc/source/whatsnew/v3.0.2.rst

@@ -22,6 +22,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression in :meth:`HDFStore.select` where the ``where`` clause on a datetime index silently returned empty results when the index had non-nanosecond resolution (:issue:`64310`)
 - Fixed regression in :meth:`Series.interpolate` where ``limit_direction="both"`` with ``limit`` greater than the Series length raised ``ValueError`` (:issue:`64322`)
+- Fixed regression in :meth:`Timedelta.round`, :meth:`Timedelta.floor`, and :meth:`Timedelta.ceil` raising ``ZeroDivisionError`` for sub-second ``freq`` (:issue:`64828`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_302.bug_fixes:

pandas/_libs/tslibs/timedeltas.pyx

@@ -2343,8 +2343,24 @@ class Timedelta(_Timedelta):
         cdef:
             int64_t result, unit
             ndarray[int64_t] arr
+            _Timedelta td_ns
 
         unit = get_unit_for_round(freq, self._creso)
+        if unit == 0:
+            unit = get_unit_for_round(freq, NPY_FR_ns)
+            if unit == 0:
+                raise ValueError("Division by zero in rounding")
+            td_ns = (<_Timedelta>self)._as_creso(NPY_FR_ns, round_ok=True)
+            arr = np.array([td_ns._value], dtype="i8")
+            try:
+                result = round_nsint64(arr, mode, unit)[0]
+            except OverflowError as err:
+                raise OutOfBoundsTimedelta(
+                    f"Cannot round {self} to freq={freq} without overflow"
+                ) from err
+            return (<_Timedelta>Timedelta._from_value_and_reso(
+                result, NPY_FR_ns
+            ))._as_creso((<_Timedelta>self)._creso, round_ok=True)
 
         arr = np.array([self._value], dtype="i8")
         try:

pandas/tests/scalar/timedelta/methods/test_round.py

@@ -185,3 +185,10 @@ def test_round_non_nano(self, unit):
         res = td.ceil("min")
         assert res == Timedelta("1 days 02:35:00")
         assert res._creso == td._creso
+
+    def test_round_freq_finer_than_resolution(self):
+        # GH#64828
+        td = Timedelta(1.0, unit="days")
+        assert td.round("100ms") == Timedelta("1 days 00:00:00")
+        assert td.floor("100ms") == Timedelta("1 days 00:00:00")
+        assert td.ceil("100ms") == Timedelta("1 days 00:00:00")