Assuming you are reporting Capital Gain/Loss for your property, according to the Canada Revenue Agency (CRA), You have a capital gain when you sell, or are considered to have sold, a capital property for more than the total of its ACB and the outlays and expenses incurred to sell the property.
Since from the Canada Revenue Agency's viewpoint your capital gain/loss starts when you sell - the date you should use is the Closing Date.
For more information, check out: Canada Revenue Agency (CRA) - Capital Gain/Loss
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