Git handling of commit times

Ed Maste emaste at
Wed May 29 14:41:27 UTC 2019

On Tue, 28 May 2019 at 23:20, Devin Teske <dteske at> wrote:
> >
> > Did anyone ever find an elegant solution to preventing the following edge-case?
> >
> > 1. Committer uses a VM but keeps it paused most of the time
> >
> > 2. Committer unpauses VM and makes a commit without first running ntpdate to correct the clock of the paused VM
> >
> > 3. Committer pushes commit with a timestamp that predates the timestamp for the previous commit on the file in-question
> >
> > In this case, GitHub renders the commits our of order. If memory serves, "git log" will show them in the proper order, but github web interface shows them in wrong order.

It seems to me there are two separate issues here - committing with an
incorrect system time, and GitHub's rendering. We should try to avoid
the first case (e.g., by hooking into VM infrastructure to have the
date set correctly after resume) where we can. I haven't actually
checked GitHub's ordering.

Git tracks the author and commit timestamps separately. They're often
the same (e.g. if I commit a change and push it to a repository) but
don't have to be - for example, if I cherry-pick change from someone
the author time will be when they originally wrote it, while the
commit time will be the time of my cherry-pick.

There are other cases where the author time of a pair of changes will
be out of order with respect to the commit ordering of those changes
-- I may have a change to a file in my tree that I haven't pushed
anywhere yet. If you then make a change to that file and I rebase, my
change will come after in the commit graph but will have an earlier
author time.

> > I think there was some talk of a hook to prevent timestamps moving backward with respect to each object being touched in a pushed commit -- is this even possible to prevent?

I believe a hook that prevents commit times from going backwards or
being ahead of the server time would serve this purpose, but I haven't
investigated in any detail. I'm also not sure off hand how GitHub,
git, and other tools use author vs commit dates. This is something
else that will need more investigation.

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