pandas-dev/pandas · error · ValueError

is not supported

Error message

{freq=} is not supported

What it means

Raised by ArrowExtensionArray._round_temporally when the offset parses successfully but its `_prefix` is not in the allow-list of units pyarrow's round_temporal supports (year, quarter, month, week, day, hour, minute, second, milli/micro/nanosecond). Examples of unsupported prefixes include business-day ('B'), semi-annual ('2QS' style edges), and custom/business offsets.

Solutions

  1. Use a supported base frequency (D, h, min, s, ms, us, ns, W, M, Q, Y) on the arrow column.
  2. Cast to datetime64[ns] (`ser.astype('datetime64[ns]')`) and round there, where business offsets are supported.

Example fix

# before
ser.dt.floor('B')
# after
ser.astype('datetime64[ns]').dt.floor('B')
Defensive patterns

Strategy: validation

Validate before calling

from pandas.tseries.frequencies import to_offset
SUPPORTED = {'Y','YS','Q','QS','M','MS','W','D','h','min','s','ms','us','ns'}
off = to_offset(freq)
if off is None or off._prefix not in SUPPORTED:
    raise ValueError(f"{freq!r} unsupported on arrow rounding")
ser.dt.floor(freq)

Type guard

def is_arrow_supported_freq(freq) -> bool:
    from pandas.tseries.frequencies import to_offset
    off = to_offset(freq)
    return off is not None and off._prefix in {'Y','YS','Q','QS','M','MS','W','D','h','min','s','ms','us','ns'}

Try / catch

try:
    out = ser.dt.floor(freq)
except ValueError as e:
    if "is not supported" in str(e):
        out = ser.astype('datetime64[ns]').dt.floor(freq)
    else:
        raise

Prevention

When it happens

Trigger: Calling `.dt.floor('B')` (business day), `.dt.ceil('SM')` (semi-month), or any custom BusinessHour/CustomBusinessDay offset on a timestamp[pyarrow] Series. `to_offset` returns a real offset, but `pa_supported_unit.get(offset._prefix)` is None.

Common situations: Using business/semiannual frequencies that work fine on datetime64[ns] but are unimplemented on the pyarrow rounding path.

Related errors

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/fd046a65af68c530. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/arrays/arrow/array.py:4224

        pa_supported_unit = {
            "Y": "year",
            "YS": "year",
            "Q": "quarter",
            "QS": "quarter",
            "M": "month",
            "MS": "month",
            "W": "week",
            "D": "day",
            "h": "hour",
            "min": "minute",
            "s": "second",
            "ms": "millisecond",
            "us": "microsecond",
            "ns": "nanosecond",
        }
        unit = pa_supported_unit.get(offset._prefix, None)
        if unit is None:
            raise ValueError(f"{freq=} is not supported")
        multiple = offset.n
        rounding_method = getattr(pc, f"{method}_temporal")
        result = rounding_method(self._pa_array, multiple=multiple, unit=unit)
        return self._from_pyarrow_array(result)

    def _dt_ceil(
        self,
        freq,
        ambiguous: TimeAmbiguous = "raise",
        nonexistent: TimeNonexistent = "raise",
    ) -> Self:
        return self._round_temporally("ceil", freq, ambiguous, nonexistent)

    def _dt_floor(
        self,
        freq,
        ambiguous: TimeAmbiguous = "raise",
        nonexistent: TimeNonexistent = "raise",

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