For example, a program that sends a token or nft to a specific address once a month.
No program on solana will be executed unless an off-chain actor submits a transaction containing an instruction for that program. There is no timer mechanism inherent to solana that will automatically execute your transactions at a later date.
You can write a program to restrict an instruction such that it can only be executed successfully once per month. The program can check the current timestamp against a previous execution to check if it's allowed to execute now. Or it could check the number of months since the previous execution and transfer the appropriate number of tokens that should be available after that number of months.
Additionally, you need to consider the incentives of the actor who submits the instruction. Does an ordinary user have reason to execute some instructions in your program already? If it fits within the compute budget, you can bundle this monthly logic along with the other logic that users routinely execute. If not, then you need to incentivize someone else to make sure the instruction is executed often enough. You could just submit a transaction once a month on your own. Or you could design your program to collect fees from ordinary users so it can pay rewards to a crank turner who runs these periodic instructions so you don't have to. You also need to let people know they can get paid for running a crank.
So, there are ways to get things to run periodically, but you need to get creative to make it happen. There are some interesting ideas that build on the primitives I have described, you can go pretty far down this rabbit hole. It has been proposed that multisig can play a role in a generic cron timer. As always, this would still require someone to turn the crank by submitting transactions to the network periodically. https://github.com/solana-labs/solana/issues/17218