If you want for whatever reason change date of the existing commit you can use the following snippet (PowerShell)
If you want to modify date of the last commit
$commitDateString = "2010-01-01T14:30:00" $env:GIT_COMMITTER_DATE = $commitDateString git commit --amend --date $commitDateString $env:GIT_COMMITTER_DATE = ""
For full list of supported date formats type
git commit --help
and look at DATE FORMATS section
If you need to change date of the non-last commit you would need to use interactive rebase first, select edit for those commits that should be changed and then apply approach shown above