A Completion Date does not need to be and should not be set as a Key Date. The confusion comes when people are considering dictionary definitions rather than contractual definitions and they are thinking that it is “key” to finish on time. They are two separate processes and used for different things and ave different contractual remedies if they are missed.
The Completion Date is the date that the Contractor has completed all of the works by to avoid paying the delay damages if X7 has been included within the contract.
A Key Date is an obligation to fulfill a certain condition by a certain date – and these would be defined in contract data part 1 as to what they are. They are to allow the Employer to manage their own interfaces, e.g. provide power to a certain area of the site by a certain date to allow his own Contractor to come in and carry out some testing. Should a Contractor miss a Key Date due to something that is their own fault/risk then any cost incurred by the Employer is recoverable from the Contractor – i.e. undefined unlimited costs. These costs could be fairly minor or significant, so could be a significant risk to the Contractor (clause 25.3).
These are different to a “Sectional Completion Date” which under X5 requires the Contractor to hand over a whole section of the works by a certain date to allow the Employer to put those elements into use (if they want to). These again would be liable to the delay damage figures outlined in X7.
I have seen an Employer define a Completion Date and then make state that a “Key Date” was the “Completion Date”. The problem comes if the Contractor is then late with that milestone – what is the financial remedy against him? Is it the delay damages in X7, the undefined/unlimited damages in clause 25.3, or even more bizarrely both (or neither). I think if nothing else this is an ambiguity that would go against the Employer anyway as they created the ambiguity but hopefully as you can see this is something the Employer does not need to do. They should just choose whether a particular milestone should be a Key Date, a Sectional Completion or the Completion Date, but not both.