code: def _validate_nbf(claims, leeway=0):
      
  
      if 'nbf' not in claims:
          return
  
      try:
          nbf = int(claims['nbf'])
      except ValueError:
          raise JWTClaimsError('Not Before claim (nbf) must be an integer.')
  
      now = timegm(datetime.utcnow().utctimetuple())
  
      if nbf > (now + leeway):
          raise JWTClaimsError('The token is not yet valid (nbf)')
docs: Validates that the 'nbf' claim is valid.
  
      The "nbf" (not before) claim identifies the time before which the JWT
      MUST NOT be accepted for processing.  The processing of the "nbf"
      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.
  
      Args:
          claims (dict): The claims dictionary to validate.
          leeway (int): The number of seconds of skew that is allowed.