Say there is a squash of 3 commits into a single commit like so:
pick abc Jan 1 stuff
squash def Jan 2 stuff
squash ghi Jan 3 stuff
pick jkl Jan 4 stuff
pick mno Jan 5 stuff
In the above, my understanding is that commits for Jan 1, 2, and 3 are squashed into a single commit. The resulting single commit is dated Jan 1 from the oldest of the 3.
Is there a way to squash in the other direction of the timestamp? Could Jan 1, 2, and 3 commits be squashed into a single commit, which is dated Jan 3?
If reordering the commits works without conflicts, then @phd's answer works right out.
Otherwise: you can edit the author date after the facts, by setting GIT_AUTHOR_DATE=... or by using git commit --date=....
For example:
# in the interactive rebase todo script:
pick abc Jan 1 stuff
squash def Jan 2 stuff
squash ghi Jan 3 stuff # <- write down the hash or short hash for 'ghi'
break # <- the rebase will pause here after squashing together the 3 commits
pick jkl Jan 4 stuff
pick mno Jan 5 stuff
# save and close
# when stopped after squashing: get the commit date from commit 'ghi'
$ git log -1 --format="%ad" ghi
$ git commit --date="<the date above>" --amend
# in one go:
$ git commit --date="$(git log -1 --format="%ad" ghi)" --amend
# if all is to your liking:
$ git rebase --continue