if any one of the week of your work period is less than 30 hrs, you would count the actual hours. More than 30 hrs counts 30 hrs.
E.g.
Week 1: 40 hrs
Week 2: 21 hrs
Week 3: 34 hrs
For the 3 weeks, you have accumulated 30+21+30 hrs = 81 hrs
As long as you accumulated 1560 hrs, you have equivalent of 1 year
In such case, you would need to prove it by all your pay slips (Not only a T4)