code: private Builder issueTime(long iat) throws InvalidClaimException {
  
          if (iat > 0) {
              claims.put(Claims.ISSUED_AT, Long.valueOf(iat));
          } else {
              // String msg = "The exp claim must be a positive number.";
              String err = Tr.formatMessage(tc, "JWT_INVALID_TIME_CLAIM_ERR", new Object[] { Claims.ISSUED_AT });
              throw new InvalidClaimException(err);
          }
          return this;
      }
docs: time in milliseconds since January 1, 1970, 00:00:00 GMT