LOCKTIME_
The BIP-65 nLockTime height/time split (Bitcoin Core's LOCKTIME_THRESHOLD): a committed unlock value below 500,000,000 is a block height; at or above it, a Unix epoch time (500,000,000 s ≈ 1985-11-05). The creation guards sit on opposite sides of it — buildDestination rejects heights at/above the threshold (exclusive ceiling: the highest committable height is one below), buildDestinationAtTime rejects timestamps below it (inclusive floor) — so every unlock value has exactly one meaning, with no gap and no overlap. isVaultMature branches on the same split. See BIP-65 and the Nexa interpreter's nLockTime handling.