crypto-marriage is a smart contract that implements the ERC721 interface and is built to represent marriages on the blockchain.
The metadata of the tokens evolve over time: creating a token requires passing a marriage date (_referenceTIme, unix timestamp), a list of URIs ( _uris, each pointing to JSON files in ERC721 Metadata JSON schema) and a list of milestones (_dayOffsets, expressed in number of days from the _referenceTime).
Every time a milestone is reached, the token's URI becomes the next element of the _uris array.
The contract is currently deployed on Polygon mainnet here.
You can view an example of a CryptoMarriage token I have minted here. The NFT is represented by a metal from the periodic table of elements and becomes a denser metal over time.