Note: changing Git history could be dangerous. Please proceed on your own responsibility.
Author date and commit date
Some StackOverflow threads told me to use this command to change last commit's date.
git commit --amend --no-edit --date="2020-10-31T00:00+09:00"
This tells Git to change last commit's date to 31st October, 2020 at 0:00 in Japan Standard Time (GTM+9:00), without editing commit message.
git log after this shows the modified date, but after pushing to remote, a GitHub repository in my case, I realised the date shown on GitHub and my local log don't match.
It turned out there are two different date records for Git history: author date and commit date. Above command only modifies the author date but not commit date. So I had to run below command to make both dates match.
git filter-branch --env-filter 'export GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"' git push -f
The right way
To avoid this confusion, what I had to run in the first place was this.
GIT_COMMITTER_DATE="2020-10-31T00:00+09:00" git commit --amend --no-edit --date "2020-10-31T00:00+09:00"