It is a rolling 12-month average, but in order to qualify for certain TM benefits (i.e. vacation accrual), your average hours must be at the minimum threshold at the point in the year when they calculate benefits eligibility (which I believe is the end of January). If you're below the minimum number of hours at that point, you will have to wait until the following year for when they calculate benefits eligibility again, regardless of how your average hours change throughout the year.
Case in point: At the end of January 2018, I was averaging 19 hours. Minimum to accrue paid vacation is 20. Even though my average increased to above 20 within a few weeks, I had to wait until April 2019 to start accruing vacation again, based on where my average hours were at the end of January 2019. At least that's how it was explained to me when I called the HROC at the time.