No - you can use the credits at any time of the day or night that you choose. They specify it that way to allow you to properly calculate exactly how many credits you will have for your trip. For example, if I am staying for a 'week long stay' and arrive on Saturday and then leave on the following Saturday - it is actually 8 days that I am on WDW property, however, I have only stayed for 7 NIGHTS at the resort. So you are only paying for the DP for 7 nights but you also only get 2 quick service credits per NIGHT per person, NOT per day that you're actually there.
Happy planning!