【日期计算】
1. 快速计算两个日期之间的日数差距
2. 根据指定的日数及起始日期，计算结束日期
3. 自订「包括首尾两日」或「不包括首天」
【Day Calculator】
1. Quickly calculate the difference between two dates.
2. Calculate End Date based on the given Period and Starting Date.
3. Allow free setting of [both days inclusive] and [exclude first day]