How can I backdate a job offer after creating, but not extending the offer?

After going to the applicant's application, clicking Create Job Offer, and then entering an incorrect date, I attempt to return to the Basic Info screen and change the date to an earlier date, as it should have been. However, I can not go back to a date before the date I originally entered on the offer. The candidate profile was active on the earlier date, as was the position on the job requisition. Any assistance would be greatly appreciated.