I have problem with timer?

Hi

this is my exp:

https://

drive.google.com/open?id=0BxuM6uiMrxpqelBxbU11d0I2dW8

i want to click buy(sprite or button) and then when i exit the game , and come back again ; system calculation time (how much exit) and minus the origin time to buy .

If any time remaining show player how much?

If time completed buy things and increase it.

and save all of this data when exit and load when back .