You may need to change the datetime of a commit in some situations. For example, some countries use daylight saving time period and because of it, some commits can have the wrong timestamp.

git commit --amend --date "Thu Feb 15 22:37:31 2018 +0300"

You should commit and then you can change the timestamp later.

References

1. https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---dateltdategt