nips/22.md

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NIP-22
======
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Event `created_at` Limits
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`draft` `optional`
Relays may define both upper and lower limits within which they will consider an event's `created_at` to be acceptable. Both the upper and lower limits MUST be unix timestamps in seconds as defined in [NIP-01](01.md).
If a relay supports this NIP, the relay SHOULD send the client an `OK` result saying the event was not stored for the `created_at` timestamp not being within the permitted limits.
Client Behavior
---------------
Clients SHOULD use the [NIP-11](11.md) `supported_nips` field to learn if a relay uses event `created_at` time limits as defined by this NIP.
Motivation
----------
This NIP formalizes restrictions on event timestamps as accepted by a relay and allows clients to be aware of relays that have these restrictions.
The event `created_at` field is just a unix timestamp and can be set to a time in the past or future. Relays accept and share events dated to 20 years ago or 50,000 years in the future. This NIP aims to define a way for relays that do not want to store events with *any* timestamp to set their own restrictions.
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_Replaceable events_ can behave rather unexpectedly if the user wrote them - or tried to write them - with a wrong system clock. Persisting an update with a backdated system now would result in the update not getting persisted without a notification and if they did the last update with a forward dated system, they will again fail to do another update with the now correct time.
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A wide adoption of this NIP could create a better user experience as it would decrease the amount of events that appear wildly out of order or even from impossible dates in the distant past or future.
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Keep in mind that there is a use case where a user migrates their old posts onto a new relay. If a relay rejects events that were not recently created, it cannot serve this use case.
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Python (pseudocode) Example
---------------------------
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```python
import time
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TIME = int(time.time())
LOWER_LIMIT = TIME - (60 * 60 * 24) # Define lower limit as 1 day into the past
UPPER_LIMIT = TIME + (60 * 15) # Define upper limit as 15 minutes into the future
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if event.created_at not in range(LOWER_LIMIT, UPPER_LIMIT):
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ws.send('["OK", event.id, False, "invalid: the event created_at field is out of the acceptable range (-24h, +15min) for this relay"]')
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```
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Note: These are just example limits, the relay operator can choose whatever limits they want.