Enforces a UTXO to remain unspendable until a certain block height is reached or a specific point in time has passed. Also known as CLTV, this op defines an absolute timelock mechanism; unlike checksequenceverify (csv), this enforces a condition where the UTXO is locked until a specified *absolute* block height or Unix timestamp. When the input item value is below 500000000, it represents a block height; otherwise, it represents a Unix timestamp. This allows for constructing contracts like payment channels for Lightning or atomic swaps, where conditions must be met after a certain period. For the operation to succeed, the value on the top of the stack must be less than or equal to the transaction's `nLockTime` field and greater than or equal to the current block height or timestamp.