Judges and the CRA prefer to count overnights. If you have substantial time with the kids during the days but not overnights, you could make a case for it. For example if you were caring for them during the weekdays and they stayed at the other parent's at night. Obviously you don't have them "zero." However if you want to make this case you shouldn't depend on being just a couple of hours over the threshold.
On the weekends the children are generally with a parent 24 hours a day, but during the week they are at school. So should weekends count double? No, in practice they don't.
When the child is in school in an intact marriage, do they stop having parents for 6 hours? No, they still have parents while at school, those parents are still their legal guardians. If a child is sick during the week, the parent is responsible for providing care. So the parent is responsible whether they are at home or at school.
Generally speaking, if a child is sick they will stay in the care of the parent they stay with the previous night; they would just stay home. So the reasonable argument is that care of the child lasts through the day until the other parent takes over for the evening (assuming a mid-week switchover.)
Really it shouldn't matter, but if someone is splitting hairs over who is responsible during working/school hours, then I would argue the parent who had them that morning. The switch happens in the evening when the child changes households.
