A couple years ago, I was one of the volunteer parents helping out at the admissions playdate session for my DS's school, and I was assigned to stand by the front door and help direct families to the right classrooms. There must have been 10 families that arrived 5-10 minutes late, and another half dozen families that arrived 15-30 minutes late. As we reached the 30-minute mark, one of the ADs came out to tell me I could leave for the day. AD said she felt sad for the families arriving so late, since their children would not get a fair chance to show their stuff in the classroom evaluations. I don't know for sure how ADs score things, but from the way she phrased her comments, my impression was that the ADs were not down-grading families who arrived late, and were not even keeping track, but that the children might indirectly suffer since they would have less time in the classrooms to interact with the playdate group. HTH.
FWIW, I'd definitely try to arrive on time for the playdate. They seem really wild and chaotic with so many children, and I'd want my child (and myself) to be as calm and organized as possible. But maybe that's just me.