I have an interest in smart contracts. For example, something simple like "bitcoin in a specific address cannot be spent until 2019-01-01".
How do I go about creating this using bitcoin-cli?
I already know how to create simple raw transactions and simple multisig raw transactions, but I can't seem to figure out how to create simple smart contracts, specifically timelocks.
I think the example you are giving is implemented by timelocks.
AFIAK the smart contracts on bitcoin can be done using rootstock (RSK). It is open source platform and RSK work as bitcoin sidechain.
You can get more information about them in their site
https://www.rsk.co/