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PSA: Git exposes timezone metadata (self.git)

submitted 2 months ago* (last edited 2 months ago) by Kalcifer to c/[email protected]

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Git records the local timezone when a commit is made [1]. Knowledge of the timezone in which a commit was made could be used as a bit of identifying information to de-anonymize the committer.

Setting one's timezone to UTC can help mitigate this issue [2][3] (though, ofc, one must still be wary of time-of-day commit patterns being used to deduce a timezone).

References

Git documentation. git-commit. "Date Formats: Git internal format". Accessed: 2024-08-31T07:52Z. https://git-scm.com/docs/git-commit#Documentation/git-commit.txt-Gitinternalformat.

It is <unix-timestamp> <time-zone-offset>, where <unix-timestamp> is the number of seconds since the UNIX epoch. <time-zone-offset> is a positive or negative offset from UTC. For example CET (which is 1 hour ahead of UTC) is +0100.
jthill. "How can I ignore committing timezone information in my commit?". Stack Overflow. Published: 2014-05-26T16:57:37Z. (Accessed: 2024-08-31T08:27Z). https://stackoverflow.com/questions/23874208/how-can-i-ignore-committing-timezone-information-in-my-commit#comment36750060_23874208.

to set the timezone for a specific command, say e.g. TZ=UTC git commit
Oliver. "How can I ignore committing timezone information in my commit?". Stack Overflow. Published: 2022-05-22T08:56:38Z (Accessed: 2024-08-31T08:30Z). https://stackoverflow.com/a/72336094/7934600
each commit Git stores a author date and a commit date. So you have to omit the timezone for both dates.

I solved this for my self with the help of the following Git alias:
```
[alias]
co = "!f() { \
    export GIT_AUTHOR_DATE=\"$(date -u +%Y-%m-%dT%H:%M:%S%z)\"; \
    export GIT_COMMITTER_DATE=\"$(date -u +%Y-%m-%dT%H:%M:%S%z)\"; \
    git commit $@; \
    git log -n 1 --pretty=\"Autor: %an <%ae> (%ai)\"; \
    git log -n 1 --pretty=\"Committer: %cn <%ce> (%ci)\"; \
}; f"
```

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[–] Kalcifer 4 points 2 months ago* (last edited 2 months ago)

I guess; otherwise the order of commits would stop making sense.

Fundamentally, Git doesn't require commits to be dated (I'm assuming some things about the protocol here — there might be something in the source code that does a check, but my point will still stand). Version control only cares about the changes from one thing to the next. The time at which the change occurred is irrelevant for this end. Things like committer names, committer emails, commit times, commit messages, etc. are QoL additions on top of what I view as the base git protocol. I'm not sure if there is an RFC which outlines Git in more detail, though. If someone is aware of such a standard, please share it.

unless you meant to auto-convert the local time to a unix timestamp, that could work. I’m overthinking stuff.

Personally, that's what I presumed they were referring to. Either that, or just storing a UTC time. Technically, I suppose storing UTC is effectively storing a timezone, but not in the sense of what this post is talking about.