nLockTime

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nLockTime is a parameter of a transaction, that, if any input indicates so (by having nSequence not equal to UINT_MAX), mandates a minimal time (specified in either unix time or block height), before which the transaction cannot be accepted into a block. If all inputs in a transaction have nSequence equal to UINT_MAX, then nLockTime is ignored.

Since BIP68, the lock time can either be absolute or relative. Given a transaction,

• If the most significant bit (1<<31) of the nLockTime field is set (if the lock time is absolute)
  • If nLockTime < 500000000
    • Specifies the block number at which this transaction can be spent
  • Otherwise
    • Specifies the UNIX timestamp at which this transaction can be spent
• Otherwise (if the lock time is relative)
  • If the 23rd bit (1<<22) is not set
    • The last 16 bits of the nLockTime field specify a relative time in units of 512 seconds. The transaction can only be included in a block if spending_tx_block_time > spent_tx_block_time + nLockTime * 512.
  • Otherwise
    • The last 16 bits of the nLockTime field specify a relative block height before which the transaction can not be included in a block. In other words it can be included if spending_tx_block_height > spent_tx_block_height + nLockTime

See Also

• lock_time in the protocol specification
• Timelock
• BIP68
• The CHECKLOCKTIMEVERIFY opcode in BIP65
• The CHECKSEQUENCEVERIFY opcode in BIP112