iotaとエラーハンドリング

    if c.VerifyExpiresAt(now, false) == false {
		delta := time.Unix(now, 0).Sub(time.Unix(c.ExpiresAt, 0))
		vErr.Inner = fmt.Errorf("token is expired by %v", delta)
		vErr.Errors |= ValidationErrorExpired
	}

	if c.VerifyIssuedAt(now, false) == false {
		vErr.Inner = fmt.Errorf("Token used before issued")
		vErr.Errors |= ValidationErrorIssuedAt
	}

	if c.VerifyNotBefore(now, false) == false {
		vErr.Inner = fmt.Errorf("token is not valid yet")
		vErr.Errors |= ValidationErrorNotValidYet
	}

const (
	ValidationErrorMalformed        uint32 = 1 << iota // Token is malformed
	ValidationErrorUnverifiable                        // Token could not be verified because of signing problems
	ValidationErrorSignatureInvalid                    // Signature validation failed

	// Standard Claim validation errors
	ValidationErrorAudience      // AUD validation failed
	ValidationErrorExpired       // EXP validation failed
	ValidationErrorIssuedAt      // IAT validation failed
	ValidationErrorIssuer        // ISS validation failed
	ValidationErrorNotValidYet   // NBF validation failed
	ValidationErrorId            // JTI validation failed
	ValidationErrorClaimsInvalid // Generic claims validation error
)