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.

Answer 1

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:

1. Generate the current date-time (in your time zone).
2. 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.