Let's say 2 computers are pushing to the same git repo, computer A has the current date time and computer B has the datetime set to 1 year earlier.
Say computer A pushed some commits, and computer B check them out and add some more commits based on the same branch and push.
*A (dated 2020-07-28 by computer A push) ----> *B (dated 2019-07-28 by computer B)
What will happen? Does git makes sure the child commits' timestamp have to be later than the parent commits?
There is no constraint on timestamps in commits.
A commit stores explicitly the id of its parent(s), and git allows a commit to have a timestamp smaller than one of its parents'
A commit actually has 2 timestamps :
Several features of git allow you to rewrite the repo's history (git rebase, git cherry-pick, git commit --amend, git filter-branch ...), and as a result, all interleavings of timestamps are possible.
When working from a single machine, you will most probably see a chronological order in the modification dates (note that it is not enforced though, if you set your system clock to "yesterday" git will happily indulge) ; when working accross several machines, git does not handle time shifts between the systems' clocks.
You can view the two timestamps with several commands :
for a single commit :
# will display timestamps rendered in human format :
git show -s --format=fuller <branch or commit id> # default is head
# will display raw unix timestamps as stored in the commit :
git cat-file -p <branch or commit id>
git cat-file -p HEAD
for each commit in a branch :
git log --format=fuller
git log --format=fuller branch1 branch2 ...
with rendering options of git log or git for-each-ref : see the --format paragraph for each of these commands
Most GUI frontends to git (e.g : gitk, git-kraken, git-extensions, sourcetree ...) will also display the two timestamps in a commit's details.