Add Reference Commits Earlier Than Reflog Remembers as a Git TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1585 TILs and counting..._
+_1586 TILs and counting..._
 
 See some of the other learning resources I work on:
 - [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -356,6 +356,7 @@ See some of the other learning resources I work on:
 - [Quicker Commit Fixes With The Fixup Flag](git/quicker-commit-fixes-with-the-fixup-flag.md)
 - [Rebase Commits With An Arbitrary Command](git/rebase-commits-with-an-arbitrary-command.md)
 - [Reference A Commit Via Commit Message Pattern Matching](git/reference-a-commit-via-commit-message-pattern-matching.md)
+- [Reference Commits Earlier Than Reflog Remembers](git/reference-commits-earlier-than-reflog-remembers.md)
 - [Remove Untracked Files From A Directory](git/remove-untracked-files-from-a-directory.md)
 - [Rename A Remote](git/rename-a-remote.md)
 - [Renaming A Branch](git/renaming-a-branch.md)

git/reference-commits-earlier-than-reflog-remembers.md

@@ -0,0 +1,34 @@
+# Reference Commits Earlier Than Reflog Remembers
+
+While preparing some stats for a recent blog post on [A Decade of
+TILs](https://www.visualmode.dev/a-decade-of-tils), I ran into an issue
+referencing chuncks of time further back than 2020.
+
+```bash
+❯ git diff --diff-filter=A --name-only HEAD@{2016-02-06}..HEAD@{2017-02-06} -- "*.md"
+warning: log for 'HEAD' only goes back to Sun, 20 Dec 2020 00:26:27 -0600
+warning: log for 'HEAD' only goes back to Sun, 20 Dec 2020 00:26:27 -0600
+```
+
+This is because `HEAD@...` is a reference to the `reflog`. The `reflog` is a
+local-only log of objects and activity in the repository. That date looks
+suspiciously like the time that I got this specific machine and cloned the
+repo.
+
+In order to access this information, I need a different approach of finding
+references that bound these points in time.
+
+How about asking `rev-list` for the first commit it can find before the given
+dates in 2017 and 2016 and then using those.
+
+```bash
+❯ git rev-list -1 --before="2017-02-07 00:00" HEAD
+17db6bc4468616786a8f597a10d252c24183d82e
+
+❯ git rev-list -1 --before="2016-02-07 00:00" HEAD
+f1d3d1f796007662ff448d6ba0e3bbf38a2b858d
+
+❯ git diff --diff-filter=A --name-only f1d3d1f796007662ff448d6ba0e3bbf38a2b858d..17db6bc4468616786a8f597a10d252c24183d82e -- "*.md"
+
+# git outputs a bunch of files ...
+```