Imagine you have a bank account, where every day 86400 is credited. This money you can afford to withdraw from the bank all day. If you can not finish all the money, the balance will be empty at the end of the day. What you do, spend all the money!
We all have a bank, and that's the time. 86400 seconds is deposited in this account every morning, and if we can not use it at this time, then it becomes empty every night, or it will not be useful at all.
If we want, we can borrow money from the bank or someone, but once the time is gone, we can not get back.
