The maths actually DOESN'T make sense. I'm sure chat must have told you but Prue was born in 1971. This was the late 1960s. This was supposed to have been Patty, the girls' mum, but the actress couldn't come back due to her schedule so they changed it to Penny instead. It's why it's Darryl's dad and not granddad. I'm not mad about the change because I understand the reason, and they made this a solid, fun episode anyway. Although I do wonder about the OG plot for this episode because I can't imagine they had Victor being a hippie lol. Would have been so cool to have seen Patty again.