Relative time locks:
Consensus code: 
GitHub
Relative locks with FCMP++ · Issue #161 · monero-project/research-lab
The legacy unlock_time field in Monero is being deprecated. It's planned that the next hard fork will enforce a value of 0 in all transactions. The...
GitHub
Relative lock - consensus code by tevador · Pull Request #507 · seraphis-migration/monero
This PR adds consensus support for relative locks (replaces #445).
Relative locks are activated from HF 18 and the relative lock time is 2160 block...