Numerical aptitude · Calendars and clocks
Calendars, made easy
💡 The idea
Whole weeks do not change the day. Only the days left over (“odd days”) move the day of the week on.
Monday→+ 100 days→= 14 weeks + 2→Wednesday
🧭 The method: 3 steps
- Count the days (finish the first month, add full months, then the days of the last month).
- Divide by 7: the remainder is the odd days.
- Move the day of the week forward by the odd days.
🎬 Every kind of question, in pictures
Pick a kind. Guess the answer first, then watch the picture work it out. Tap any step to see it in the picture.
Loading the pictures…
✏️ Worked examples
Example 1. Today is Monday. Day after 100 days?
- 100 = 14 × 7 + 2
- Monday + 2 = Wednesday
Example 2. Days after 20 March is 10 May?
- Rest of March: 31 − 20 = 11
- April: 30
- May: 10
- 11 + 30 + 10 = 51 days
Example 3. Is 1900 a leap year?
- It divides by 4…
- but it is a century year, so it must divide by 400
- 1900 ÷ 400 is not whole → not a leap year
⚠️ Where people go wrong
- Counting both end days when the question says “after”.
- Forgetting that 1900 and 2100 are NOT leap years (2000 is).
- Months are not all 30 days.
📌 Remember
- Odd days: year 1, leap year 2
- 100 yrs 5, 200 yrs 3, 300 yrs 1, 400 yrs 0
- 0 = Sunday, 1 = Monday, …
- 31 → 3, 30 → 2, 28 → 0, 29 → 1