date_add ( timestamp with time zone, interval [, text ] ) → timestamp with time zone → timestamp with time zone将一个
interval 加到 timestamp with time zone 上,并根据第三个参数命名的时区,或在省略该参数时根据当前TimeZone设置,计算时刻和夏令时调整。双参数形式等价于 timestamp with time zone + interval 操作符。