Thanks for suggesting to contact us, which for the benefit of other's reading this, Mirco did.
So, to answer the questions, the Twitter Search API returns the last set of Tweets based on a search (either 1 week's worth of Tweets or 1,500 Tweets, which ever is reached first) - this is a limitation of the Twitter API, not QVSource.
As such you can't define a start date in the way that you would like.
You can set the since_id to the last Tweet that you retrieved, which helps QVSource be more efficient in it's calls to the Twitter API, so you don't make more requests than it needs to.
You can read more about these two points on our dedicated documentation site: