The days of the week repeat every 7 days. So if you know only the remainder when the number of days is divided by 7, you can immediately find the day of the week without turning the calendar page by page. Calculating patterns that repeat at regular intervals like this is called modular arithmetic. Change today's day and the number of days ahead with the sliders and see how it works.
How to calculate it: Move forward from today by the remainder when the number of days ahead is divided by 7. Since 100 ÷ 7 = 14 remainder 2, after 14 weeks (exactly 98 days) you are back on the same day of the week, and you only need to move 2 more days.
A clock works the same way: A clock repeats every 12 hours (modulo 12), so if you want to know what time it will be 100 hours from now, you only need to calculate the remainder when 100 is divided by 12. It works exactly the same way as calculating the day of the week.