Transaction Lock Time (nLockTime)

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The final field of a transaction contains a 4-byte nLockTime value.

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If nLockTime < 500,000,000, the integer value is interpreted as the block height. Otherwise the integer is interpreted as the UNIX timestamp. If any of the transaction’s inputs have a non-final nSequence number, nLockTime represent the earliest time at which the transaction can be timestamped in a block.

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https://wiki.bitcoinsv.io/index.php/NLocktime_and_nSequence#nLockTime

Note that the nLockTime of a transaction is ignored if all the transaction inputs have the maximum nSequence value (UINT_MAX). In this case, the transaction may be included in any valid block, irrespective of its nLockTime value.