Lecture Notes: Calendar - Lesson 1

Introduction

• Topics Covered: Calendar (main topic), Clock (mentioned briefly)
• Importance: Essential for exams like SSC, RRB, IT sector interviews, state govt exams
• Approach: Focus on traditional methods before shortcuts
• Traditional vs. Shortcut: Traditional methods help understand concepts and handle exceptions; shortcuts are faster but less reliable for all cases

Ordinary Year vs. Leap Year

• Ordinary Year: 365 days
• Leap Year: 366 days
• Determination:
  • Normal Years: Year divisible by 4 (e.g., 2020 is a leap year, 2019 is ordinary)
  • Century Years: Year divisible by 400 (e.g., 1600 is a leap year, 1800 is ordinary)
• February: 28 days in an ordinary year; 29 days in a leap year

Week and Odd Days Concept

• Ordinary Year: 52 weeks + 1 odd day
• Leap Year: 52 weeks + 2 odd days
• Odd Days: Extra days beyond complete weeks
  • E.g., 23 days = 3 weeks + 2 odd days

Examples of Determining Leap or Ordinary Year

• Example Years:
  • 2016, 2012 (Leap Years)
  • 1947, 1945, 1923 (Ordinary Years)
  • Century Years like 1800, 1600, 2000

Calculating Odd Days

• Formula: Total days = Weeks + Odd Days
  • Examples:
    • 23 days → 3 weeks + 2 odd days
    • 8 days → 1 week + 1 odd day
    • 50 days → 7 weeks + 1 odd day
• Odd Day Chart: Mapping extra days to week days
  • 0: Sunday, 1: Monday, 2: Tuesday, ..., 6: Saturday

Odd Days in Multiple Years

• Grouping Years: Calculate leap and ordinary years in a given span
  • Examples:
    • First 20 years: 5 leap years, 15 ordinary years → 25 odd days → 3 weeks + 4 odd days → Thursday
    • First 40 years: 10 leap years, 30 ordinary years → 50 odd days → 7 weeks + 1 odd day → Monday
    • First 100 years: 24 leap years, 76 ordinary years → 124 odd days → 17 weeks + 5 odd days → Friday
    • First 400 years: Check century years by 400 → 0 odd days → Sunday
• Formulas: (easy to remember)
  • 20 years: 4 odd days
  • 40 years: 1 odd day
  • 100 years: 5 odd days
  • 200 years: 3 odd days
  • 300 years: 1 odd day
  • 400 years: 0 odd days (multiples of 4 centuries)

Revision

• Ordinary Year vs. Leap Year: Divisible by 4 (not century) or divisible by 400 (century)
• Week Calculation: Days divided by 7 to calculate weeks + odd days
• Odd Day Chart: Remember mapping for easy calculations

Practice and Application

• Importance of Practice: Understanding through practice is crucial for solving calendar problems effectively
• Examples for Practice: Determine leap years, calculate odd days, and find which day a date falls on

Thank you for watching and learning!