Unix Timestamp

A Unix timestamp records time as the number of seconds elapsed since 00:00:00 UTC on January 1, 1970, a reference moment called the Unix epoch. It is the standard time format across computing, and every bitcoin block header contains a 4-byte Unix timestamp set by the miner who produced the block.

How it works

Bitcoin cannot consult a trusted clock, so it enforces loose honesty instead: a block's timestamp must be greater than the median timestamp of the previous eleven blocks and no more than two hours ahead of a node's network-adjusted time. Within that window miners have slack, and timestamps occasionally even run backward between consecutive blocks. The protocol needs only approximate time, since timestamps feed the difficulty adjustment that retargets mining every 2,016 blocks to hold the ten-minute average. The famous year 2038 problem, when signed 32-bit timestamps overflow, does not threaten bitcoin, whose unsigned field runs until 2106, by which point a soft fork can extend it.

Why it matters

The timestamp is the humble mechanism behind bitcoin's grandest claim, that a decentralized network can maintain its own clock and its own history without any authority. The genesis block's timestamp, and the newspaper headline embedded beside it, fixed bitcoin's birth at January 3, 2009 in a way no one can revise. Gold's history lives in archives that can burn or lie; bitcoin's lives in a ledger that carries its own dates.