Just quoted a thrifty car at BOS

pickup at Thursday 11:45pm: $39.xx per day
pickup at Thursday 12:00pm: $19.xx per day

what a big difference 15 minutes make

You mean 11:45 AM and 12 NOON?

Weekend rates always start at 12 NOON on Thursday at all major car rental companies. Some have a Saturday night stay, some a Friday night stay to get the weekend rate.

I've even had Dollar quote the weekend rate at 12 noon Thursday with a Friday return.