Where is a broadcasted TX located at before it's nLocktime reached? in mempool or not? I have tried a lot can't figure out. Help please.
Such transactions are located only with the transaction creator. Nodes generally do not keep in their mempool transactions that cannot be immediately mined.
further to Achow's answer, here an extract from Andreas' bitcoin bible ("Mastering Bitcoin, 2nd edition"):
If nLocktime is nonzero and below 500 million, it is interpreted as a block height, meaning the transaction is not valid and is not relayed or included in the blockchain prior to the specified block height. If it is above 500 million, it is interpreted as a Unix Epoch timestamp (seconds since Jan-1-1970) and the transaction is not valid prior to the specified time. Transactions with nLocktime specifying a future block or time must be held by the originating system and transmitted to the bitcoin network only after they become valid. If a transaction is transmitted to the network before the specified nLocktime, the transaction will be rejected by the first node as invalid and will not be relayed to other nodes.