Nbf means "Not Before" and is a Reserved Claim Name defined in RFC7519 Section 4.1.5

The "Nbf" (not before) JSON Web Token Claim identifies the time before which the JWT MUST NOT be accepted for processing. The processing of the "Nbf" JSON Web Token Claim requires that the current date/time MUST be after or equal to the not-before date/time listed in the "Nbf" claim.

Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew. Its value MUST be a number containing a NumericDate value.

Use of this claim is OPTIONAL.

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