Changes that deviate from a contract can only be made in agreement with the talent. If you agree with the talent to change the end time, he or she should report the time you have agreed, which you then approve.
If you do not agree on a new end time, the talent is entitled to the time and remuneration that is in the contract. If no changes are made, it is the contract period you are billed for.
Keep in mind that the information contained in the contract is definitive, however, the information that first appears in the gig's job description can be estimated time and adjusted before the contract is sent out.