git log --since date string parsing

Question

I was looking through some logs in a submodule today (somewhat furiously) and typed some errors into the dates in the command

git log --since="2019-12-31" --oneline | wc -l
      90

and instead of typing that I typed,

git log --since="2019-12=31" --oneline | wc -l
    2147

I was alarmed by the difference is count which is why I was able to notice the error immediately. However, I'm unclear as to what the behavior is in this case. Is this a bug or is this expected behavior for git log?

I would expect an error in parsing but it looks like the logs go back to January or February of 2019.

Does this answer your question? [Specification for syntax of git dates](https://stackoverflow.com/questions/14023794/specification-for-syntax-of-git-dates) — Adrian Cornish, Feb 02 '20 at 01:58
Did you check the actual output? Are you sure it isn't 2147 lines of "bad date" type errors? — Adrian Cornish, Feb 02 '20 at 01:59
@AdrianCornish: It's not that, git is actually outputting the history, I tried it. — John Zwinck, Feb 02 '20 at 02:07

John Zwinck · Accepted Answer · 2020-02-02T12:14:07.677

It appears that Git's date parsing just stops when it gets to a part it can't understand. For example "2015-13" is understood as "2015", as is "2015-4444".

In your case, it's treating "2019-12=31" as "2019". And then a second weird "feature" kicks in: --since 2019 does not mean --since 2019-01-01, it means "since today's date in 2019". So the meaning of your --since 2019-12-31 will actually change throughout each year when you run it!

What seems to be happening is:

Generate the current date-time (in your time zone).
Replace the parts you specified in --since, ignoring the month and day part if the parser cannot decide how to parse it.

Note that even the simple and "obviously correct" git log --since 2019-12-31 does not really do what one might expect, because it shows commits since the current time on that day, not since midnight. So unless you specify a full date-time string, there will always be some dependency on the time when the command is executed.

thanks for clarifying and for the link to approxidate c code, very interesting! Marking as correct. — Lucas Roberts, Feb 07 '20 at 17:38

git log --since date string parsing

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