Absolute Timelock

An absolute timelock prevents a Bitcoin transaction from being included in a block until a fixed point in the future — either a specific block height or a specific calendar time. It is the original timelock mechanism, expressed through the transaction's 4-byte nLockTime field and enforceable inside a locking script with OP_CHECKLOCKTIMEVERIFY.

How nLockTime works

The nLockTime value is interpreted against a single threshold: values from 0 to 499,999,999 are read as a block height, and values from 500,000,000 to 4,294,967,295 are read as a Unix timestamp. The block-height range covers thousands of years of future blocks, while the timestamp range runs to the year 2106. For nLockTime to actually take effect, at least one input's nSequence must be set to 0xfffffffe or lower; otherwise the field is ignored and the transaction is treated as final.

What it is good for

Absolute timelocks underpin inheritance schemes, escrow refunds, and savings vaults where a coin must be unspendable until a known date or height. Embedding the constraint in the scriptPubKey via CLTV makes it trustless: no party can move the funds early regardless of who holds the keys. Time-based absolute locks are evaluated against Median Time Past, which is why they confirm about an hour later than a naive reading of the timestamp suggests.

For deadlines measured from an output's confirmation instead of a fixed point, see Relative Timelock. The opcode that enforces absolute locks in script is OP_CHECKLOCKTIMEVERIFY (CLTV).