I am new to programming. At this moment i am working on a module called licensing for a tool. I had a question on how to validate a license using date. i am trying to use system date - registered date.

requirement is

if a license is valid for 150 days right now(22-oct-2009) and if the user comes and changes his System date after 100 days(29-jan-2010) to (22-oct-2009) then license should not be extended to 150 days it should be 50 days only. how should i clear this. I think the BIOS time cant be changed, If i am mistaken please correct me. if i am correct then how to get that BIOS time. Or please help me in getting a solution.