[amenod]

So how does the system know what time it is? Host time is not reliable. Genuinly curious.

This is the reason why Ethereum contract usually set start time, end time and similar in block numbers. Block number is reliable while timestamp is just a local time of the miner that mined the transaction in which the contract call was included.

[TD-Linux]

Timestamps in the block header. These timestamps are used to regulate the proof of work to maintain a block interval, so in the end both measures have the same time accuracy.