TERM_DEF // TRANSACTION / ABSOLUTE_LOCKTIME
ABSOLUTE
LOCKTIME
LOCKTIME
locktime/">Absolute Locktime. A locktime referring to a specific block-height/">block height or timestamp.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_ABSOLUTE_LOCKTIME_IS
Absolute Locktime — at a glance
TRANSACTION
Absolute Locktime is part of the SEQUENCE / LOCKTIME sub-family of transaction-level concepts. A 4-byte tx-level field specifying the earliest absolute time (block height < 500M, or unix time ≥ 500M) at which the tx can be mined. A locktime referring to a specific block height or timestamp.
Why it exists
DESIGN
Some contracts (e.g. Lightning HTLC timeout paths) require a transaction to be valid only AFTER a certain time. Absolute locktime gives the protocol a way to say "this is a future-dated tx, don't mine it before time T".
HOW_IT_WORKS
Mechanism
HOW IT WORKS
nLockTime is checked against either the current chain tip's height or its median-time-past (BIP-113). If nLockTime is non-zero, at least one input must have sequence < 0xffffffff or the locktime is ignored.
1. Wallet selects UTXOs whose total value covers the spend amount + estimated fee (coin selection).
2. Wallet builds the transaction body: version, inputs (each with prev txid + vout + sequence), outputs (each with value + scriptPubKey), locktime.
3. Wallet computes the sighash for each input (which parts of the tx the signature commits to — controlled by the SIGHASH flag).
4. Wallet signs each input with the right private key. Witness/scriptSig is populated with the resulting signatures + pubkeys.
5. Tx is broadcast to peers. Mempool propagation: tens of seconds globally.
6. A miner includes it in a block. Confirmation count grows by 1 per block; after ~6 the tx is effectively final.
WORKED_EXAMPLE
Absolute Locktime — SEQUENCE / LOCKTIME
EXAMPLE
nLockTime values:
0 no locktime; tx valid immediately
500,000 block height — valid once chain reaches 500k blocks
1,700,000,000 unix timestamp — valid once chain median time past ≥ this
0x500000000 (≥ 500M) interpreted as unix time, not block height
For locktime to apply: at least one input must have sequence ≠ 0xffffffff.
Median Time Past (BIP-113): the median timestamp of the last 11 blocks, used for time-based locktimes to prevent miner timestamp games.
KEY_PROPERTIES
ATOMIC
A transaction is either fully accepted into a block or fully rejected. There is no partial spend.
IMMUTABLE INPUTS
A UTXO can only ever be spent once. After that, it is permanently consumed.
NO BALANCES
IRREVERSIBLE
After ~6 confirmations (~1 hour), reversing the tx would require more proof-of-work than the honest network produces.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- address-reuse/">Address reuse degrades privacy — every reuse links more of your UTXOs together publicly. Modern wallets generate a fresh address per receive.
- Fee estimation matters: under-pay and your tx sits in the mempool for hours; over-pay and you tip the miner more than necessary. Use a fee estimator.
- "Change outputs" must go back to a fresh address you control. A missing output/">change output sends the difference to the miner as fee — a known footgun.
- RBF (Replace-By-Fee) lets you re-broadcast a tx with a higher fee. Useful for stuck txs but means a 0-confirmation tx is never truly final.
RELATED_CONCEPTS
Other terms from Transaction — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
Absolute Locktime
A locktime referring to a specific block height or timestamp.
Transaction (Tx)
A signed payload spending one or more UTXOs and creating new ones; every state change in Bitcoin is a tx.
Raw Transaction
The hex-serialized bytes of a transaction, ready to broadcast or analyze.
Transaction ID (TXID)
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Input
A reference to a previous output being spent, plus the data (scriptSig/witness) authorizing the spend.
Output
An (amount, scriptPubKey) pair created by a transaction; spendable later by a tx whose input references it.
UTXO (Unspent Transaction Output)
An output that hasn't been spent yet; your "balance" is the sum of UTXOs you can sign for.