No, really it is two questions:
1. OK, I arrive at Heathrow tonight and put a seven day travel card on my Oyster, but I do not use my card until sometime tomorrow. Do I correctly assume the seven days begins with first usage and not with purchase.
2. Say I first use the card at noon on Saturday. When are my seven days up? At the end of Friday, the seventh day, at noon the following Saturday, or some other time?