Re: Why do dates in refspecs require the reflog?

[Michael Haggerty <mhagger@alum.mit.edu>]

On 12/30/2014 09:12 PM, Robert Dailey wrote:
> I was surprised to find today that the following didn't work as expected:
> 
> $ git show master@{"1 year ago"}:Path/To/File.h
> 
> It gave me the following warning which did not make the problem obvious at all:
> warning: Log for 'master' only goes back to Mon, 8 Sep 2014 12:08:44 -0500.
> 
> What it is trying to tell me (and I only found this out after a lot of
> poking and prodding around Google) is that I can't refer to a commit
> by date if it extends beyond the range of the reflog.
> 
> I don't understand why this requirement is in place. Each commit has a
> time stamp, so in theory it should be possible to calculate an
> absolute date from the duration specified in the refspec and grab the
> commit from `git log` from that.
> 
> I can only assume I'm oversimplifying things and that there is a good
> reason for this. If I can't get this to work, how can I view the
> contents of a file at a specified relative time frame as I have done
> above?

The syntax you are using,

    master@{"1 year ago"}

tells you "what commit was the master branch in this clone pointing at
one year ago?" What you presumably expected is "What is the last [1]
commit that is *currently* on the master branch that was committed at
least one year ago?"

The command to ask your question is something like

    git log --date-order --before='1 year ago' -1 master

There is yet a third similar question: "What is the last commit that is
currently on the master branch that was *authored* at least one year
ago? Please note that this question is even subtler, because author
timestamps are often out-of-order even on a single branch (whereas
commit timestamps are usually in order). I don't know of a simple git
command to answer this question.

Michael

[1] Note that "last" is a bit subtle, because the history of "master"
probably contains multiple branches with interleaved dates. So depending
on your needs, you might want to add an option like "--first-parent"

-- 
Michael Haggerty
mhagger@alum.mit.edu