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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ZipFile tests fail in negative timezone offsets on reproducible build environments","articleBody":"# Bug report\n\n### Bug description:\n\n#### Background\n - Relates to https://github.com/python/cpython/issues/73894\n - Reproducibility for ZipFile introduced in https://github.com/python/cpython/issues/91279\n - See https://reproducible-builds.org/ and https://reproducible-builds.org/docs/buy-in/ for why this is a good thing to have in general, and https://reproducible-builds.org/docs/source-date-epoch for basic information on `SOURCE_DATE_EPOCH` specifically.\n\n#### Bug\nIf building on a reproducible environment such as NixOS, and your local timezone has a negative UTC offset, a large number of the ZipFile tests fail due to a underflow of the DOS date value:\n - Zips by default store file timestamps as a DOS date and a DOS time, They are unsigned and 16-bits wide. \n - NixOS sets environment variable `SOURCE_DATE_EPOCH` to `315532800` (1980-01-01 00:00:00 UTC) to specifically support programs and standards assuming DOS time (https://github.com/NixOS/nixpkgs/pull/89794), of which zip is one.\n - In recent ZipFile updates, `SOURCE_DATE_EPOCH` is read from if it exists. If it exists, it's not taken as UTC and is instead converted to the system's local time:\n   - https://github.com/python/cpython/blob/8d490b368766ee7b28d2ccf47704b1d4b5f1ea23/Lib/zipfile/__init__.py#L625-L628\n - If your local time has a negative UTC offset, this ends up subtracting from the minimum possible DOS date value, which underflows to what would be a negative DOS date.\n - In tests, `ValueError` is raised anytime a file is written to a ZipFile, because the DOS date under reproducible systems is not in the range `0 \u003c= date \u003c= 65535`.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-134264\n* gh-150137\n* gh-150138\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ctrlaltf2","@type":"Person","name":"ctrlaltf2"},"datePublished":"2025-05-19T18:06:19.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/134261/cpython/issues/134261"}