NEC3 Short Form of Contract Programme

Assuming there has been nothing written into the Works Information regarding submission of a programme i.e. format, content etc. the ECSC 2013 says that the Contractor submits a forecast of the date of Completion to the Employer each week and then the Employer decides the date of Completion and then certifies it to the Contractor. What guidance is there on how the Employer decides this and what happens if the Contractor and Employer disagree?
Many thanks.