May I ask another one?
Then how does the time expire works? Given that each node have different time?
There's two ways, UNIX time stamp or Block height.
With the UNIX code, the nodes will check the transactions in the block against the timestamp of the block. If the timestamp of the transaction is lower than the timestamp of the block, the whole block becomes invalid. Blocks can be up to 2 hours inaccurate.
For the block height, the transactions that is in the block must at least meet the block height of the block that it is in. If not, it is invalid.