Being a donor for quite a while, I noticed that the privileges are working by the calendaristic system. I firmly consider that this system is not fair for the donor. If I donate for one month privileges and I cannot use slsk because of various reasons, objective ones (I'm in hospital or I have an accident etc), I cannot make use of my privileges and plus, I cannot recover the days lost. I would consider that a system based strictly on the time a donor uses slsk is more fair than the current system. I donate for one month privileges, I use slsk 1 day, then only one day to be taken from that full month.
You might say that this system is not fair for you, but I suppose that all the users that donate to slsk, they use slsk frequently, otherwise is not smart to donate and not use the service.