def validate_nbf(max_clock_skew = 0)
      max_clock_skew ||= 0

      nbf = time_claim("nbf")
      if nbf && nbf > (Time.now + max_clock_skew)
        raise Sandal::ClaimError, "The token is not valid yet."
      end
    end
def validate_nbf(max_clock_skew = 0)
      max_clock_skew ||= 0

      nbf = time_claim("nbf")
      if nbf && nbf > (Time.now + max_clock_skew)
        raise Sandal::ClaimError, "The token is not valid yet."
      end
    end
Validates the not-before claim.

 @param max_clock_skew [Numeric] The maximum clock skew, in seconds.
 @return [void].
 @raise [Sandal::ClaimError] The "nbf" claim is invalid, or the token is
   not valid yet.