
Fertilisation to delivery is usually 60 days, the pregnancy calendars allow extra days as bitch will usually stand at ovulation so doesn't actually conceive for further 2-3 days.
I progesterone test and know when they ovulated and mate 2-3 days post ovulation, I therefore expect labour at day 60/61 and would be unhappy about leaving a bitch longer. If vet in doubt about need for section you can progesterone test again and if level below 2ng/l she should be in labour.