Date Validator with Leap Year Detection

🔬 Understanding Leap Year Logic

Gregorian Calendar Rules:

// Leap year detection algorithm:

bool isLeapYear = (year % 400 == 0) || 
                  (year % 4 == 0 && year % 100 != 0);

// Breaking it down:

// Condition 1: Divisible by 400
(year % 400 == 0)
// Examples: 1600, 2000, 2400 → Leap years

// OR

// Condition 2: Divisible by 4 BUT NOT by 100
(year % 4 == 0 && year % 100 != 0)
// Examples: 2004, 2008, 2020 → Leap years
// But NOT: 1700, 1800, 1900 → Not leap years

Test Cases for Leap Years:

📆 Days in Each Month

Memory Rhyme:

Thirty days hath September,
April, June, and November.
All the rest have thirty-one,
Except for February alone,
Which has twenty-eight days clear,
And twenty-nine in each leap year.

🔄 Validation Flow Diagram

Input: day/month/year
         ↓
┌─────────────────────────┐
│ Step 1: Validate Year   │
│ Is 1900 ≤ year ≤ 9999?  │
└─────────────────────────┘
         ↓ NO → Invalid
         ↓ YES
┌─────────────────────────┐
│ Step 2: Validate Month  │
│ Is 1 ≤ month ≤ 12?      │
└─────────────────────────┘
         ↓ NO → Invalid
         ↓ YES
┌─────────────────────────┐
│ Step 3: Determine       │
│ Max Days for Month      │
│                         │
│ ┌─ Month == 2?         │
│ ├─── YES → Leap Year?  │
│ │         ├─ YES → 29  │
│ │         └─ NO  → 28  │
│ ├─ Month in {4,6,9,11}?│
│ │         └─ YES → 30  │
│ └─ Else  → 31          │
└─────────────────────────┘
         ↓
┌─────────────────────────┐
│ Step 4: Validate Day    │
│ Is 1 ≤ day ≤ maxDays?   │
└─────────────────────────┘
         ↓ NO → Invalid
         ↓ YES
       Valid!

⚠️ Common Edge Cases to Test

🔑 Key Concepts Learned

• Complex Validation: Multi-step validation with dependencies
• Leap Year Algorithm: Understanding Gregorian calendar rules
• Conditional Logic: Nested if-else for month-based day validation
• Ternary Operator: Compact leap year to max days conversion
• Early Return: Exiting function when validation fails
• Input Parsing: Reading slash-separated date format
• Logical Operators: Combining conditions with && and ||

🌍 Real-world Applications

• Form Validation: Birthdate, appointment date validation in web apps
• Database Integrity: Ensuring valid dates before insertion
• Booking Systems: Hotel, flight, event date validation
• Payroll Systems: Validating work periods and payment dates
• Age Verification: Calculating valid birthdates for age restrictions
• Calendar Applications: Preventing invalid event dates
• Financial Software: Transaction date validation
• Medical Records: Ensuring valid appointment and prescription dates
• Travel Systems: Departure/arrival date validation

💪 Extension Challenges

• Level 1: Calculate day of week for valid dates (Zeller's algorithm)
• Level 2: Calculate days between two valid dates
• Level 3: Add time validation (full datetime validator)
• Level 4: Support multiple date formats (DD/MM/YYYY, MM/DD/YYYY, YYYY-MM-DD)
• Level 5: Add next/previous day calculator
• Level 6: Validate historical dates (handle calendar changes)
• Level 7: Create age calculator from birthdate
• Bonus: Build a complete calendar system with holidays

💡 Practice Tips

• Test All Edge Cases: Use the edge case table above
• Understand Leap Years: Practice with years 1900, 2000, 2004, 2100
• Learn the Rhyme: Memorize "Thirty days hath September..."
• Trace Validation: Draw flowchart and follow for each test case
• Test February: Special attention to Feb 28 vs 29
• Boundary Testing: Try day 0, day 32, month 0, month 13
• Study Calendars: Look at real calendars to understand patterns
• Research History: Learn about Gregorian calendar adoption