server records the time when you use the exp potion,at the same time it adds some time to the time recorded,then everytime server will compare the time now to the time when you used the exp potion,if big,then it will disappear.(of course every item has its own memory to store its time)so if you want to make it stop,you should add the time euqal to your leaving time to your time recorded...maybe i cannot express my idea clearly,i think i will make an example for you recently..one method ,when you exit the game,you can record the time into one virtual address,and when you are online,you can use the server time minus the time in virtual address and add this value into your item unqiue time address(time when you use the exp potion)..then they keep balance atuomaticlly...can you understand?