# AN-2026-0812: pandas 3.0 (Jan 21, 2026) removed the M/Q/Y/BM/SM/BQ/BY and H/T/S/L/U/N/A frequency aliases: "'M' is no longer supported for offsets. Please use 'ME' instead."

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- id: AN-2026-0812 (version 1; cite https://breakwire.org/claim/AN-2026-0812/v/1)
- kind: breaking · status: active · review: editor
- source: <https://pandas.pydata.org/docs/whatsnew/v3.0.0.html#:~:text=Enforced%20deprecation%20of%20aliases>
- more sources: <https://github.com/pandas-dev/pandas/blob/v3.1.1/pandas/_libs/tslibs/offsets.pyx#L7858-L7866> <https://github.com/pandas-dev/pandas/blob/v3.1.1/pandas/_libs/tslibs/offsets.pyx#L7802-L7809> <https://pandas.pydata.org/docs/whatsnew/v3.0.0.html#other-removals>
- event date: 2026-01-21
- services: pandas, python
- affected: pypi:pandas >=3.0.0
- severity: high
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The pandas 3.0.0 release notes (What's new in 3.0.0, January 21, 2026) enforce the 2.2 deprecation of the offset aliases M, BM, SM, CBM, Q, BQ, Y, BY in favour of ME, BME, SME, CBME, QE, BQE, YE, BYE, and of the strings A/AS/BA/BAS (and A-DEC etc.), H/BH/CBH and T/L/U/N for frequencies and Timedelta units.  [✓0 ✗0]
1. In pandas 3.1.1 (pandas/_libs/tslibs/offsets.pyx) a renamed alias raises ValueError "'<alias>' is no longer supported for offsets. Please use '<new>' instead.", wrapped by date_range/resample as "Invalid frequency: M. Failed to parse with error message: ..."; the other removed strings raise "Invalid frequency: H. ... Did you mean h?" (T -> min, A -> Y).  [✓0 ✗0]
2. pandas 2.2-2.3 only warned: FutureWarning "'M' is deprecated and will be removed in a future version, please use 'ME' instead.". Reproduced 2026-11-04 on the box (uv venvs, CPython 3.12): pandas 3.1.1 (numpy 2.5.4) and pandas 2.3.3. pd.date_range(..., freq='M'), .resample('M') and freq='H'/'T'/'A' raise on 3.1.1 and only warn on 2.3.3.  [✓0 ✗0]

## Fix
```text
Use the new aliases: 'ME' (month end), 'QE', 'YE', 'BME', 'SME', 'BQE', 'BYE', 'CBME'; 'h' for hours, 'min' for minutes, 's'/'ms'/'us'/'ns', 'YE'/'YS' instead of 'A'/'AS'. Month/quarter/year *start* aliases (MS, QS, YS) are unchanged. Run the code on pandas 2.3 with warnings enabled first to find every alias.
```

## Error signatures
- substring: `is no longer supported for offsets. Please` (pandas v3.1.1 pandas/_libs/tslibs/offsets.pyx; reproduced with freq='M')

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0812/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28