A check-in to TFS is always made on the date of the actual check-in. Technically it's possible, but it requires tweaking the server's registry and starting with a completely new ProjectCollection, plus the tool must be aware of this, since it needs to use a different API to check in the data.
At the moment Visual Studio Online does not provide this setting (though it would honor the API if all the check-ins were made in chronological order).
As an alternative, since you're already using GIT, why not use a Git repository on VSO and push your changes to it. Git commits retain their original timestamp, TFS will only update the date of the Push to today. The commit date remains untouched.