NIP-40 defines an expiration tag that tells relays when an event should be deleted.

How It Works

Events include an expiration tag with a Unix timestamp:

["expiration", "1734567890"]

After this time, relays should delete the event and refuse to serve it.

Why It Matters

  • Ephemeral content that should disappear after a set time
  • Time-limited offers or announcements
  • Listing expiration in marketplaces (e.g., Shopstr)
  • Reducing relay storage requirements

Expiration is a retention hint, not a revocation system. It helps align relay behavior around stale content, but it does not guarantee erasure once another relay, client, or archive has already copied the event.

Trust and Security Notes

  • Relays are not required to honor expiration (but most do)
  • Clients should not rely on expiration for security-critical content deletion
  • Once content is fetched by another client, it may be cached or re-published
  • Expiration does not hide that an event existed. Event ids, quotes, or off-relay copies may still survive after the timestamp passes

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