Author and Committer Dates in Git – An Obscure Bug
The Context
While preparing a Git repository for my tutorial “The Disappearance of J. T. Womblegast — A Git Tutorial” for the Agile Testing Days 2024, I wanted to set the commit and author date for commits explicitly.
In most cases, these dates are id, but they can be different, as the author and committer _can_ different people (For details about the commit details, see the extensive Git documentation for commits). One interesting details about the dates: The author date can be set with the command line option --date , while the committer date can only be set usung an environment variable.
How to set the dates
I chose to set the author date using the command line option, and the environment variable GIT_COMMITTER_DATE for the commiter date.
When there is a change in the Git index (or staging area), here’s who to commit it whit given date and time info:
> GIT_COMMITTER_DATE="Sun Aug 4 11:14:08 2024 +0200" git commit -m "Demo commit" --date "Wed Jul 24 19:09:11 2024 +0200" [main (root-commit) cbaea5e] Demo commit Date: Wed Jul 24 19:09:11 2024 +0200 1 file changed, 1 insertion(+) create mode 100644 wahoodie
All is good: I created a file with some content, added it to the staging area, and committed it.
Varying the dates
Let’s be some more daring with the commit dates and go a few years into the future and past. Add some more text to the file ‘wahoodie’ I used before, add the change to the Git index and commit:
> echo "more text" >> wahoodie > git add . > GIT_COMMITTER_DATE="Sat Jan 1 00:00:00 2000 +0200" git commit -m "Another commit" --date "Tue Jan 19 2038 03:14:08 UTC" [main 133c21b] Demo commit Date: Tue Jan 19 03:14:08 2038 +0000 1 file changed, 1 insertion(+)
It still looks good.
When it’s not working
Now let’s for far into the past:
> echo "another line" >> wahoodie > git add . > GIT_COMMITTER_DATE="Wed 11 Oct 1634 22:47:38 +0000" git commit -m "Updates file" --date "Tue Jan 19 2038 03:14:08 UTC" fatal: invalid date format: Wed 11 Oct 1634 22:47:38 +0000
Huh? Is something wrong with the (committer) date? It looks OK, but itcould be the weekday that’s causing trouble. Let’s try to commit a change using the wrong weekdays:
> echo "even more text" >> wahoodie > git add . > GIT_COMMITTER_DATE="Mon Aug 4 11:14:08 2024 +0200" git commit -m "Demo commit" --date "Sun Jul 24 19:09:11 2024 +0200" [main e2e848e] Demo commit Date: Wed Jul 24 19:09:11 2024 +0200 1 file changed, 1 insertion(+) stephan@seaside ~/dev/garble-git-repo/tmp/wahoodie ᚠ main > git log --format="Author Date: %ad %nCommitter Date: %cd %h" -1 Author Date: Wed Jul 24 19:09:11 2024 +0200 Committer Date: Sun Aug 4 11:14:08 2024 +0200 e2e848e
The given date is used and the offered week day was corrected. Things look OK when leaving out the weekday altogether:
> echo "one more line" >> wahoodie > git add . > GIT_COMMITTER_DATE="Aug 4 11:14:08 1987 +0200" git commit -m "further update" --date "Jul 24 2042 19:09:11 +0200" [main 696dc2b] further update Date: Thu Jul 24 19:09:11 2042 +0200 1 file changed, 1 insertion(+) > git log --format="Author Date: %ad %nCommitter Date: %cd %h" -1 Author Date: Thu Jul 24 19:09:11 2042 +0200 Committer Date: Tue Aug 4 11:14:08 1987 +0200 696dc2b
The real cause of the error message
After some experimenting with the date & talking to folks on the Git Discord, I figured the the epoch is the (lower) boundary for dates in Git. Let’s use the following shell script (‘demo-date-format-issue.sh’):
git init tmp cd tmp echo "Some content" > non-empty-file.txt git add . GIT_COMMITTER_DATE="31 Dec 1969 23:59:60 +0000" git commit -m "Commit 1" --date "12 Dec 1969 23:59:60 +0000" GIT_COMMITTER_DATE="01 Jan 1970 00:00:00 +0000" git commit -m "Commit 1" --date "01 Jan 1970 00:00:00 +0000"
Running it gives the following output:
> ./demo-date-format-issue.sh Initialized empty Git repository in /Users/stephan/dev/garble-git-repo/tmp/.git/ fatal: invalid date format: 31 Dec 1969 23:59:60 +0000 [main (root-commit) 9c0b65a] Commit 1 Date: Thu Jan 1 00:00:00 1970 +0000 1 file changed, 1 insertion(+) create mode 100644 non-empty-file.txt
Aha! The first commit attempt fails – and gives a misleading error messgage: The date format is perfectly valid. Apparently Git can’t process a date before 1st January 1970 00:00:00 UTC.
Remarkably, the next date that might cause trouble, ‘2038-01-19T03:14:08+00:00’, doesn’t. Instead the last future date & time at which a commit is possible is ‘2099-12-31T23:59:59+00:00’, as the following example code shows:
echo "More content" > non-empty-file.txt git add . GIT_COMMITTER_DATE="01 Jan 2100 00:00:00 +0000" git commit -m "Commit 2" --date "01 Jan 2100 00:00:00 +0000" GIT_COMMITTER_DATE="31 Dec 2099 23:59:60 +0000" git commit -m "Commit 2" --date "31 Dec 2099 23:59:60 +0000"
The output of this is:
fatal: invalid date format: 01 Jan 2100 00:00:00 +0000
I don’t understand why the year change from 2099 to 2100 is the upper boundary for dates Git will process. To me this seems to be a bit limiting since it’s ‘just’ ≈75½ years until then. (If you know why this limit has been chosen, please share.)
In any case: Printing an error message that claims a date format to be invalid, when it’s not is a bug. I said ‘obcure bug’ in the title. 😃