def validate_exp(max_clock_skew = 0)
      max_clock_skew ||= 0

      exp = time_claim("exp")
      if exp && exp <= (Time.now - max_clock_skew)
        raise Sandal::ExpiredTokenError, "The token has expired." 
      end
    end
def validate_exp(max_clock_skew = 0)
      max_clock_skew ||= 0

      exp = time_claim("exp")
      if exp && exp <= (Time.now - max_clock_skew)
        raise Sandal::ExpiredTokenError, "The token has expired." 
      end
    end
Validates the expires claim.

 @param max_clock_skew [Numeric] The maximum clock skew, in seconds.
 @return [void].
 @raise [Sandal::ClaimError] The "exp" claim is invalid, or the token has
   expired.