The calculation is done when a transaction or investment is made. The threshold tells the companies whether they need to make a notification under the act, which is simply advising the government that a transaction is occurring. Alternatively, if they're over the threshold, then they must go through an application for review, at which point the Minister of Industry is entitled to review whether the investment is of net benefit. It is a snapshot at the time of the investment.
On May 21st, 2013. See this statement in context.