NIP-3 specifies how to use OpenTimestamps to timestamp events, proving they existed prior to a point in time: https://github.com/nostr-protocol/nips/blob/master/03.md
But it may also be useful to prove events were created _after_ a point in time. The easiest way to do that would be to add a tag with a recent block hash to every event you create.
pkt / Peter Todd
npub1ej…3ndrm
2023-02-24 17:34:56
Author Public Key
npub1ej493cmun8y9h3082spg5uvt63jgtewneve526g7e2urca2afrxqm3ndrmPublished at
2023-02-24 17:34:56Event JSON
{
"id": "58d70e71861de7143ae98eb17a4792d57faa7ca9ceab5626bfccf9fcac194367",
"pubkey": "ccaa58e37c99c85bc5e754028a718bd46485e5d3cb3345691ecab83c755d48cc",
"created_at": 1677256496,
"kind": 1,
"tags": [],
"content": "NIP-3 specifies how to use OpenTimestamps to timestamp events, proving they existed prior to a point in time: https://github.com/nostr-protocol/nips/blob/master/03.md\n\nBut it may also be useful to prove events were created _after_ a point in time. The easiest way to do that would be to add a tag with a recent block hash to every event you create.",
"sig": "59fffed3997fce1fcf51ca32efe959573e515ab8e95c5e0f0479d9f0b728fb73ed5b22d069b593577de16fb840afe5d54e19773720265d23578683d78c6396e7"
}