The default date in the Revision History object that is placed on the Master Layouts calls on the automatic Date. This system date just pulls today's date. If you want to use your own date, you need to add a new filed in the Issue Scheme. You can do this in your template and/or when you create a new issue.
Then you edit the Revision History object on your master layout(s) to call this new date field.
Cheers,
Link.