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Tue, 6 Oct 2026 10:37:28 -0400 (EDT) From: Junio C Hamano To: "Devi Srinivas Vasamsetti via GitGitGadget" Cc: git@vger.kernel.org, Devi Srinivas Vasamsetti Subject: Re: [PATCH] commit: warn when a new commit is dated before its parent In-Reply-To: (Devi Srinivas Vasamsetti via GitGitGadget's message of "Mon, 05 Oct 2026 15:09:58 +0000") References: Date: Tue, 06 Oct 2026 07:37:27 -0700 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain "Devi Srinivas Vasamsetti via GitGitGadget" writes: > From: Devi Srinivas Vasamsetti > > Git writes whatever the clock says into the commit object. This can > be problematic because history traversal assumes commit dates are > non-decreasing. For example, "git log --since" stops walking at the > first commit older than the cutoff, so an out-of-order date hides > the commits behind it. It might be annoying, but the value of such a warning is unclear. If you clone from an upstream repository, you might find that the commit at the tip was made on a machine with a clock set far in the future. When you try to make a commit on top of it, what are you supposed to do? Wait for a year so your commit is newer than the tip? Ask the committer to correct their clock, redo the commit, and force-push? It also does not help if the commit at the tip of the cloned branch has a timestamp in the past, but is a child of a commit with an incorrect timestamp. Nobody would receive a warning, yet --since may still stop prematurely. Stepping back a bit, suppose you clone from upstream and obtain a HEAD dated 24 hours ago, HEAD~1 dated 72 hours ago, and HEAD~2 dated 48 hours ago. A command like git log --since=50.hours may stop without showing HEAD~2, but if you suspect that some clocks are skewed, there is no way to determine the correct output from these timestamps anyway. It is possible that HEAD~1 has an incorrect timestamp and was actually written 30 hours ago, meaning all three commits should be shown. Alternatively, HEAD~2 might have an incorrect timestamp and was written 80 hours ago, in which case showing neither HEAD~1 nor HEAD~2 is correct. The moral of the story is that --since or any other time-based option cannot be fully reliable, as you cannot force everyone to run with a correctly synchronized clock. If you truly need to know the ancestral relationship between commits, you should avoid these options and use topology-based ones instead.