BUG: DatetimeIndex.is_year_start and DatetimeIndex.is_quarter_start always return False on double-digit frequencies

doc/source/whatsnew/v3.0.0.rst

@@ -419,6 +419,7 @@ Interval
 Indexing
 ^^^^^^^^
 - Bug in :meth:`DataFrame.__getitem__` returning modified columns when called with ``slice`` in Python 3.12 (:issue:`57500`)
+- Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 -
 
 Missing

pandas/_libs/tslibs/fields.pyi

@@ -16,7 +16,7 @@ def get_date_name_field(
 def get_start_end_field(
  dtindex: npt.NDArray[np.int64],
  field: str,
- freqstr: str | None = ...,
+ freq_name: str | None = ...,
  month_kw: int = ...,
  reso: int = ..., # NPY_DATETIMEUNIT
 ) -> npt.NDArray[np.bool_]: ...

pandas/_libs/tslibs/fields.pyx

@@ -210,7 +210,7 @@ cdef bint _is_on_month(int month, int compare_month, int modby) noexcept nogil:
 def get_start_end_field(
  const int64_t[:] dtindex,
  str field,
- str freqstr=None,
+ str freq_name=None,
  int month_kw=12,
  NPY_DATETIMEUNIT reso=NPY_FR_ns,
 ):
@@ -223,7 +223,7 @@ def get_start_end_field(
  ----------
  dtindex : ndarray[int64]
  field : str
- frestr : str or None, default None
+ freq_name : str or None, default None
  month_kw : int, default 12
  reso : NPY_DATETIMEUNIT, default NPY_FR_ns
 
@@ -243,18 +243,17 @@ def get_start_end_field(
 
  out = np.zeros(count, dtype="int8")
 
- if freqstr:
- if freqstr == "C":
+ if freq_name:
+ if freq_name == "C":
  raise ValueError(f"Custom business days is not supported by {field}")
- is_business = freqstr[0] == "B"
+ is_business = freq_name[0] == "B"
 
  # YearBegin(), BYearBegin() use month = starting month of year.
  # QuarterBegin(), BQuarterBegin() use startingMonth = starting
  # month of year. Other offsets use month, startingMonth as ending
  # month of year.
 
- if (freqstr[0:2] in ["MS", "QS", "YS"]) or (
- freqstr[1:3] in ["MS", "QS", "YS"]):
+ if freq_name.lstrip("B")[0:2] in ["MS", "QS", "YS"]:
  end_month = 12 if month_kw == 1 else month_kw - 1
  start_month = month_kw
  else:

pandas/_libs/tslibs/timestamps.pyx

@@ -579,15 +579,15 @@ cdef class _Timestamp(ABCTimestamp):
  if freq:
  kwds = freq.kwds
  month_kw = kwds.get("startingMonth", kwds.get("month", 12))
- freqstr = freq.freqstr
+ freq_name = freq.name
  else:
  month_kw = 12
- freqstr = None
+ freq_name = None
 
  val = self._maybe_convert_value_to_local()
 
  out = get_start_end_field(np.array([val], dtype=np.int64),
- field, freqstr, month_kw, self._creso)
+ field, freq_name, month_kw, self._creso)
  return out[0]
 
  @property

pandas/core/arrays/datetimes.py

@@ -145,8 +145,12 @@ def f(self):
  kwds = freq.kwds
  month_kw = kwds.get("startingMonth", kwds.get("month", 12))
 
+ if freq is not None:
+ freq_name = freq.name
+ else:
+ freq_name = None
  result = fields.get_start_end_field(
- values, field, self.freqstr, month_kw, reso=self._creso
+ values, field, freq_name, month_kw, reso=self._creso
  )
  else:
  result = fields.get_date_field(values, field, reso=self._creso)

pandas/tests/indexes/datetimes/test_scalar_compat.py

@@ -328,3 +328,11 @@ def test_dti_is_month_start_custom(self):
  msg = "Custom business days is not supported by is_month_start"
  with pytest.raises(ValueError, match=msg):
  dti.is_month_start
+
+ def test_dti_is_year_quarter_start_doubledigit_freq(self):
+ # GH#58523
+ dr = date_range("2017-01-01", periods=2, freq="10YS")
+ assert all(dr.is_year_start)
+
+ dr = date_range("2017-01-01", periods=2, freq="10QS")
+ assert all(dr.is_quarter_start)