Timestamp

A timestamp is a date-and-time label. Each block in a blockchain carries one, showing roughly when it was created. Because each block is chained to the one before it, the timestamps line up into an ordered history that's very hard to fake.

That's useful beyond money: people use blockchains to prove a document existed at a certain time, without revealing what it says. Bitcoin's genesis block carries a famous one, a newspaper headline that pins down its earliest possible date.