It is well established that the general measure of damages for breach of contract is the amount of damages that will, so far as money can, place the aggrieved party in the same position as if the wrong had not been done.
The focus is on the injured party’s loss and on the measure of compensation required to restore it to the position that it would have been in had the contract been performed.
With respect to the appropriate date for the assessment of damages, the presumption is that damages, including those for loss of a business or opportunity, should generally be assessed as of the date of breach: Johnson v. Agnew,  A.C. 367 (H.L.), at pp. 400-401.
The general presumption that damages will be assessed as of the date of breach may be subject to exceptions where fairness requires it. However, this presumption should not be easily displaced; any deviation from it must be based on legal principle. As the British Columbia Court of Appeal recently noted in Dosanjh v. Liang, 2015 BCCA 18 (CanLII), 380 D.L.R. (4th) 137, at para. 55:
[T]he presumption that contract damages are to be assessed as of the date of the breach is not so easily displaced. It is important that the law in this area be predictable, and such predictability is not served by allowing judges unbounded discretion as to the date for assessment of damages.
This general presumption should only be displaced in special circumstances, such as, for example, where no market exists to replace undelivered shares at the date of breach: Kinbauri, at para. 126; or in relation to “[s]ome classes of property, including shares, whose value is subject to sudden and constant fluctuations of unpredictable amplitude, and whose purchase is not lightly entered into”: Asamera Oil Corp. Ltd. v. Sea Oil & General Corp.,  1 S.C.R. 663, at pp. 664-65.