commit: warn when a new commit is dated before its parent

[ysai258]

RFC. Adds an advice.clockSkew warning when git commit creates a commit dated earlier than one of its parents, which usually means the system clock is wrong and can make git log --since silently skip commits.

Rationale, scope and open questions are in the commit message and this comment.

[gitgitgadget]

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[ysai258]

This is an RFC. A warning of this shape has been suggested several times over the years without landing, so I would rather hear whether the idea is wanted at all before polishing it.

The problem

Git writes whatever the clock says and validates nothing. A commit dated years in the future, or earlier than its own parent, is accepted silently, and git fsck --strict does not object either — badDate and badDateOverflow are purely syntactic checks.

That would be harmless if history traversal did not assume commit dates are non-decreasing, but it does:

$ git log --pretty='%cd %s' --date=short
2026-09-25 C3 - inside the window
2026-09-01 C2 - outside the window
2026-09-20 C1 - inside the window

$ git log --pretty='%cd %s' --date=short --since=2026-09-13
2026-09-25 C3 - inside the window

C1 is inside the window and silently missing, because --since stops walking at C2. This is understood — 9669778 (revision: add "--since-as-filter" option, 2022-07-19) added an opt-in traversal mode for exactly it — but nothing tells the person whose clock caused the problem, at the point where it is still cheap to fix.

I hit this on a repository of my own after moving the system clock to test date-dependent behaviour. Nothing warned me, and by the time I noticed, the dates were part of the object names.

What this does

Warns at commit time when the new commit's date precedes a parent's, gated on a new advice.clockSkew.

hint: the new commit is dated 2026-09-13 06:00:00 +0530,
hint: which is earlier than its parent, dated 2026-09-25 10:00:00 +0000.
hint: This usually means the system clock is wrong.
hint: Commands that walk history in date order, such as
hint: "git log --since", may skip commits as a result.
hint: Disable this message with "git config set advice.clockSkew false"

Scope, deliberately narrow

• Warn, never refuse. Only the committer can tell whether their clock or the parent's is wrong.
• The commit being created and its parents, nothing else. Skew between machines is ordinary in a distributed system. This fires only when one repository's own history steps backwards.
• git commit only. Merges and replayed history go through other paths, where non-monotonic dates are frequently legitimate. I took this to be the lesson of the objection to checking every incoming commit on the receive side.

Open questions for reviewers

1. Is this wanted at all? Several people have concluded over the years that it is a good idea, and equally that it may not be worth the effort.
2. Is hint: the right channel? There is a fair objection that stderr advice gets buried among progress output.
3. Should fsck grow an INFO-tier check for the "far ahead of now" case as well? That cannot compare against parents — fsck_commit() deliberately never loads parent objects — so it would be a separate, weaker check.

Tests are in t7502; t7502, t7501, t7500 and t0018 all pass.

[dscho]

@ysai258 you will want to move the rationale back into the first comment, which will be sent as a "cover letter" (for single-patch contributions such as yours, the rationale is inserted between the diffstat and the diff). Many of the frequent reviewers like their Git mailing list so much that they will be angry if you try to lead them elsewhere. I am not condoning that behavior, but I want to set you up for success by avoiding that preventable friction.

Also, to mark it as an RFC, simply use GitHub's "Convert to draft" function before /submiting. (And you might want to test first via /preview, which will send the email only to you, even if the email you receive should claim the Git mailing list to be in Cc:.)

[dscho]

/allow

[dscho]

/allow

Hrm. That failed. But interestingly, https://www.githubstatus.com/ says:

[dscho]

I tried to re-run, but...

GitHub Status still says everything's dandy.

[gitgitgadget]

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