Scheduling of payments doesn't happen through time in blockchain. When you specify the nLocktime it is about when your transaction gets included in a block which has a height higher than the one you have specified in the nlocktime as far as GreenAddress is considered. You can create a transaction, sign them using your private key and specify the nLockTime to a future block. Till that time (i.e till the mentioned block is mined) the node considers your transaction as invalid if you try to broadcast them and none of the miners will include your locked transaction in a block. All you can do, is sign the tx and store them and the wallet broadcast it at the right time once the certain block has reached. Time Locking is done for locking our funds for children/heirs or similar, but it isn't really advisable to do so.
For instance if you create a transaction and enter nlocktime as 600,000 which will be mined sometime in the future, your transaction will be considered invalid till the network reaches the 600,000 block and later then they would be included in a future block based on the priority of the fee you have specified. But nlock transactions have an added advantage, where you can cancel the transaction before they are included in a future block by spending the same input with another transaction.
No wallet has it as far as I know.
Not true. Green Address wallet uses the nLockTime feature to schedule the transactions for a future block. Here, the Greenaddress wallet stores your fund in a 2of2 multi-sig wallet so that both you and greenaddress require to sign the transaction if they want to be broadcasted to the network before the lock time ends. You could even try Coinb.in for creating and spending a time locked transaction. Check TryNinja
post on how to carry out one.
What if you specify the transaction to a far future block?
This is not really advisable to do so. Btc is receiving updates and code changes quite very often, so if you lock your coins for a longer period and a soft fork or similar occurs it might be difficult for you to spend your old coins. Not really sure about this, but this might happen.
If you are willing to create a locked transaction, try reading this :
https://steemit.com/bitcoin/@daan/how-to-create-time-locked-transactions-with-bitcoin-free-bitcoins-inside