Tohoku and south Hokkaido pass

Hi,
My wife and I would like to book two regional travel passes for travel sometime during January. We are visiting our son in Hokkaido and plan to stay with him for between ten days and two weeks before travelling by train through northern japan using the regional travel pass.. I went to book them but found that I have to put in a start date, but at the moment, we don’t know exactly when we will be using the tickets. We know that once activated, the tickets need to be used for 6 consecutive days. We just want to be a little bit flexible about when we start those days…is that possible ?