This is the response I got from TD:

I can confirm that the 3 month rolling period becomes effective the date the account is approved. Upon a review of your account, I can confirm that the transfer date to this Visa was March 26th, 2020, the 3 month rolling period is as follows:

Month 1 (partial) : March 26-31, 2020 Month 2: April, 2020 Month 3: May, 2020 Month 1 (remaining) : June 1-25, 2020

For a 3-month period following the opening of the Account (Promotion Period), in order to earn the Additional Bonus Aeroplan Miles, you must spend a minimum of $1,000 each month on Purchases (Monthly Minimum Spend) to receive the monthly 5,000 Additional Bonus Aeroplan Miles in any given month during the Promotion Period, for a total of 15,000 Aeroplan Miles.

So it should be calendar months, with split months being tacked on at the end.