The employer elects to calculate the damages suffered at the time of the delay and claim those from the contractor. Such damages follow the Hadley v Baxendale principle:
Where two parties have made a contract that one of them has broken, the damages which the other party ought to receive in respect of such breach of contract should be such as may fairly and reasonably be considered to be arising naturally, ie according to the usual course of things from such breach of conduct itself, or such as may reasonably be supposed to have been in the contemplation of both parties at the time they made the contract, as the probable result of the breach of it.