As for your date question: If you set your survey to be anonymous, then there will be no real date stamp stored at the survey details to keep anonymity. Instead a fake date (1980--01-01 00:00:00 if I remember correctly) is stored.
You can work around that by creating a question of type equation and adding the following as question text:
{date("Y-m-d")}.
You can also set the question to be hidden at advanced question settings -> always hide this question.

This will save the current date at a (hidden) question together with the results.

Mazi's example is a workaround. I would just put a word of caution out there that given the right scenario a hidden date stamp could turn the survey from anonymous to confidential. Just a thought.

Yes, I agree, this is important. On the one hand I want people to know that this is going to be anonymous - so that we can get as honest feedback as possible. On the other hand grouping by dates and following developments in such a way is important to us. Another consideration I suppose is in context of a place where a given number of people come in every week; there always could be some way how a dedicated person theoretically could correlate new records in the database with people visiting that day. In this way it is a bit different to a survey on internet scenario. Hence I would think that what is nearly equally important her is policy. We need to think exactly how we would treat the data and then explain it clearly and stick to it.