I just read the Guidelines for the shop I did today. It does not address how late you can begin within the assigned time period. It does say that you have to stay in the dining area for 15 minutes AFTER you receive your order.
Shopping Southeast Pennsylvania, Delaware above the canal, and southwestern NJ since 2008
Yes, you have to start within the time that is given. They are not like Seelvel where you have to be there 20 minutes before the end time of your shop. Dinner is 5 P to 8 P but do make sure you are there by 7:40 so you have time to get the meal during the dinner hour. It took me 40 minutes to get something yesterday and then you have to explain why it took so long. Maybe it is because they do not pay that well and people are tired of it. Eventually, we will pass on the low ball offers and will only do it for the free meal every now and then.