The locktime field is a 4-byte value that specifies the earliest time a transaction can be added to the blockchain.

Let's examine our transaction to understand this better:

In our example transaction, we can see the locktime value is 00000000. This means:

• The transaction is valid immediately (can be included in the next block)
• No timelock restrictions are applied
• This is the most common locktime value used in Bitcoin transactions

Locktime Values

The interpretation of the locktime value depends on its range:

For example:

• Locktime of 650000 means the transaction is invalid until block 650000
• Locktime of 1640995200 means the transaction is invalid until January 1, 2022 (Unix timestamp)

Implementation Example

Here's how you might parse a transaction's locktime:

def parse_locktime(raw_tx: bytes, offset: int = 0) -> tuple[int, int]:
    """
    Parse locktime from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (locktime, new_offset)
    """
    # Read 4 bytes for locktime
    locktime_bytes = raw_tx[offset:offset + 4]
    
    # Convert to integer (little-endian)
    locktime = int.from_bytes(locktime_bytes, 'little')
    
    return locktime, offset + 4

Use Cases

1. Time-Delayed Transactions

Locktime can be used to create transactions that only become valid in the future:

def create_future_tx(block_height: int) -> bytes:
    """Create a transaction valid only after specific block height"""
    tx = Transaction()
    # ... add inputs and outputs ...
    
    # Set locktime to future block
    tx.locktime = block_height
    
    # Must set sequence < 0xffffffff for locktime to be enforced
    for tx_in in tx.inputs:
        tx_in.sequence = 0xfffffffe
        
    return tx.serialize()

2. Escrow Transactions

Locktime can be used in escrow services where funds should only be released after a certain time:

def create_escrow_tx(release_time: int) -> bytes:
    """Create a transaction that releases funds at specific time"""
    tx = Transaction()
    # ... add inputs and outputs ...
    
    # Set locktime to future timestamp
    tx.locktime = release_time
    
    # Enable locktime enforcement
    for tx_in in tx.inputs:
        tx_in.sequence = 0xfffffffe
        
    return tx.serialize()

Validation Rules

When validating a transaction with locktime:

1. If all input sequences are 0xffffffff, locktime is ignored
2. Otherwise:
   • If locktime < 500000000: Compare against current block height
   • If locktime ≥ 500000000: Compare against median time past (MTP)
3. Transaction is invalid if current time/height is less than locktime