What is the resolution of Git's commit-date or author-date timestamps?

The resolution of Git commit/author dates is 1 second, which, as pointed out by Alexey Ten and Edward Thomson, is also the resolution of Unix timestamps.

An interesting experiment that you can conduct is to

• create a commit, and
• amend it very quickly, without changing anything (not even the commit message).

As you may know, amending a commit actually creates a new commit. Normally, the new commit would have a different timestamp, and, therefore, a different commit ID from that of the first commit. However, you can write a script that creates the commit and amends it within the same system-clock second (with a bit of luck!), thereby producing a commit whose hash is the same as the first commit's.

First, set things up:

$ mkdir testGit
$ cd testGit
$ git init

Then write this to a script file (called commitAmend.sh below)

#!/bin/sh

# create content and commit
printf "Hello World.\n" > README.md
git add README.md
git commit -m "add README"
git log

# amend the commit
git commit --amend --no-edit
git log

and run it:

$ sh commitAmend.sh
[master (root-commit) 11e59c4] add README
 1 file changed, 1 insertion(+)
 create mode 100644 README.md
commit 11e59c47ba2f9754eaf3eb7693a33c22651d57c7
Author: jub0bs <xxxxxxxxxxx>
Date:   Fri Jan 30 14:25:58 2015 +0000

    add README
[master 11e59c4] add README
 Date: Fri Jan 30 14:25:58 2015 +0000
 1 file changed, 1 insertion(+)
 create mode 100644 README.md
commit 11e59c47ba2f9754eaf3eb7693a33c22651d57c7
Author: jub0bs <xxxxxxxxxxx>
Date:   Fri Jan 30 14:25:58 2015 +0000

add README

Same timestamp, same hash!

It is one second; while git-show is prettifying the output a bit, you can see the raw commit information using the git-cat-file command. For example:

% git cat-file commit HEAD
tree 340c0a26a5efed1f029fe1d719dd2f3beebdb910
parent 1ac5acdc695b837a921897a9d42acc75649cfd4f
author Edward Thomson <[email protected]> 1422564055 -0600
committer Edward Thomson <[email protected]> 1422564055 -0600

My witty comment goes here.

You can see indeed that this is a Unix timestamp with a resolution of 1 second.