datetime.strptime: %j, %z, non-padded fields, lowercase %b/%p fail, and every mismatch escapes as .NET FormatException instead of ValueError

[antonsynd]

Found by /verify-implementation plan-0ca7b7 (2026-09-26), measured @ 61bf20f with the built sharpyc; pre-existing at BASE acd1d40 unless noted.

datetime.datetime.strptime translates the python format into a .NET ParseExact format (Datetime.cs TranslateStrptimeFormat). Python (3.12) vs Sharpy:

call	python3	sharpy
strptime("2021 003", "%Y %j")	datetime(2021, 1, 3, 0, 0)	FormatException
strptime("2021-01-03 +0530", "%Y-%m-%d %z")	aware, offset +05:30	FormatException
strptime("2021-1-3", "%Y-%m-%d")	datetime(2021, 1, 3, 0, 0)	FormatException
strptime("jan 3 2021", "%b %d %Y")	datetime(2021, 1, 3, 0, 0)	FormatException
strptime("2021-01-03 1pm", "%Y-%m-%d %I%p")	datetime(2021, 1, 3, 13, 0)	FormatException
strptime("x", "%Y") inside except ValueError	ValueError: time data 'x' does not match format '%Y'	uncaught System.FormatException (unhandled exception)

The last row is the worst one: an expected failure isn't catchable by the python spelling. Class contract: strptime parses by python's directive grammar, the same way strftime now does (0eca2b3's Strftime.cs is the formatter twin), and a mismatch raises ValueError with python's message. Sibling of #2019.

[antonsynd]

Batching 2026-09-30 @ db8e05b — charter .claude/plans/remediation-round-2026-09-20-batching.md §3.30, P35 — culture invariance + datetime fidelity (order N4).

Contract: strptime parses by python's directive grammar (the Strftime.cs formatter twin exists since 0eca2b3) and a mismatch raises ValueError with python's text — never an uncatchable FormatException. Lane / reach: runtime-throw · program. Cells per the issue's table × {catchable by except ValueError}. Class tracker: #2056. No seam hypothesis — the plan measures the seam.