docs(datetime): add real-life applications of Calendar and GregorianCalendar in Java

What
- Documented practical use cases of java.util.Calendar and java.util.GregorianCalendar.
- Added code examples for:
  1. Event Scheduling (checking weekends/holidays before booking).
  2. Payroll & HR systems (last working day salary disbursement).
  3. Banking/Finance (loan repayment adjustment for weekends/holidays).
  4. Ticket Booking & Reservations (handling unavailable/holiday dates).
  5. Inventory Management (expiry and replenishment tracking).
  6. Employee Leave & Attendance Tracking.
  7. Astrology & Festival Calculations (e.g., Easter Sunday).
  8. Subscription Renewals (reminder before expiry).

Why
- Calendar API is widely used in legacy Java apps pre–Java 8 for date-time manipulation.
- Many enterprise systems (payroll, banking, reservations) still depend on Calendar.
- Helps developers understand real-world scenarios where Calendar and GregorianCalendar are applied.

Comparisons
- Added a table contrasting legacy Calendar vs. modern java.time API.
- Highlighted issues with mutability, thread-safety, and API complexity.
- Recommended java.time (LocalDate, ZonedDateTime) for modern Java 8+ projects.

Real-Life Relevance
- Shows how to handle scheduling conflicts (weekends/holidays).
- Demonstrates business-critical use cases like payroll, billing, and subscriptions.
- Reinforces importance of correct date handling in enterprise software.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section26DateandTimeAPI/src/DateandTimeAPI/Real-Life Applications of Calendar and GregorianCalendar in Java.txt

@@ -1,19 +1,19 @@
-Real-Life Applications of Calendar and GregorianCalendar in Java
+Real-Life Applications of Calendar and GregorianCalendar in Java:
 
-The **Calendar API (`java.util.Calendar` and `java.util.GregorianCalendar`)** is widely used in
+The Calendar API (`java.util.Calendar` and `java.util.GregorianCalendar`) is widely used in
 legacy Java applications for handling **dates, times, and scheduling** before Java 8 introduced `java.time`.
 
-Although modern applications use `java.time`, some systems still rely on the `Calendar` API. Here are some **real-life applications**:
+Although modern applications use `java.time`, some systems still rely on the `Calendar` API.
 
----
+Here are some real-life applications:
+
+1. Event Scheduling Systems 🗓️
+- Used in meeting schedulers, calendars, and reminders.
+- Example: Booking an appointment in a hospital or corporate meeting.
 
-### 1. Event Scheduling Systems 🗓️
-- Used in **meeting schedulers, calendars, and reminders.
-- Example: Booking an appointment in a **hospital** or **corporate meeting**.  
+🔹 Example Use Case:
+- Check if a selected date is a weekend or holiday before scheduling an event.
 
-🔹 **Example Use Case:**  
-- Check if a selected date is a **weekend** or **holiday** before scheduling an event.  
-```java
 import java.util.Calendar;
 import java.util.GregorianCalendar;
 
@@ -29,17 +29,16 @@ public class EventScheduler {
         }
     }
 }
-```
 
 ---
 
-### **2. Payroll & HR Systems** 💰  
-- Used to determine **payroll cycles**, calculate **working days**, and set **payment schedules**.  
-- Example: Calculating **salary disbursement** on the last working day of the month.  
+2. Payroll & HR Systems:
+- Used to determine payroll cycles, calculate working days, and set payment schedules.
+- Example: Calculating salary disbursement on the last working day of the month.
+
+🔹Example Use Case:
+- Find the last working day of the month for salary processing.
 
-🔹 **Example Use Case:**  
-- Find the **last working day** of the month for salary processing.  
-```java
 import java.util.Calendar;
 import java.util.GregorianCalendar;
 
@@ -51,17 +50,16 @@ public class PayrollSystem {
         System.out.println("Salary will be processed on: " + payrollDate.getTime());
     }
 }
-```
 
 ---
 
-### **3. Banking and Finance Applications** 🏦  
-- Used for **loan processing, interest calculations, and credit card billing cycles**.  
-- Example: Checking if a loan repayment date falls on a **bank holiday**.  
+3. Banking and Finance Applications
+- Used for loan processing, interest calculations, and credit card billing cycles**.
+- Example: Checking if a loan repayment date falls on a bank holiday.
+
+🔹 Example Use Case:
+- If a loan repayment date is on a Sunday, shift it to the **next working day.
 
-🔹 **Example Use Case:**  
-- If a loan repayment date is on a **Sunday**, shift it to the **next working day**.  
-```java
 import java.util.Calendar;
 import java.util.GregorianCalendar;
 
@@ -75,25 +73,24 @@ public class LoanRepayment {
         System.out.println("Loan repayment due date: " + dueDate.getTime());
     }
 }
-```
 
 ---
 
-### **4. Ticket Booking & Reservation Systems** 🎟️  
-- Used in **airlines, railways, and hotel reservations** to check availability.  
-- Example: Finding the **next available seat** for a **flight/train**.  
+4. Ticket Booking & Reservation Systems
+- Used in airlines, railways, and hotel reservations** to check availability.
+- Example: Finding the next available seat for a flight/train.
 
-🔹 **Example Use Case:**  
+🔹 Example Use Case:
 - If a user requests a ticket on a **public holiday**, show the **next available date**.  
 
 ---
 
-### **5. Inventory Management & Stock Rotation** 📦  
-- Used in **warehouses** to track product **expiry dates** and **stock replenishment**.  
-- Example: Checking if a **perishable item** expires soon.  
+5. Inventory Management & Stock Rotation
+- Used in warehouses** to track product expiry dates and stock replenishment.
+- Example: Checking if a perishable item expires soon.
+
+🔹Example Use Case:
 
-🔹 **Example Use Case:**  
-```java
 import java.util.Calendar;
 import java.util.GregorianCalendar;
 
@@ -109,31 +106,30 @@ public class InventoryManager {
         }
     }
 }
-```
 
 ---
 
-### **6. Employee Leave and Attendance Tracking** 🏢  
-- Used to check **employee attendance** and **leave approval**.  
-- Example: Calculating the **number of leave days taken** in a year.  
+6. Employee Leave and Attendance Tracking:
+- Used to check employee attendance and leave approval.
+- Example: Calculating the number of leave days taken in a year.
 
-🔹 **Example Use Case:**  
-- Check if an employee has taken leave **on a public holiday** or a **weekend**.  
+🔹Example Use Case:
+- Check if an employee has taken leave on a public holiday or a weekend.
 
 ---
 
-### **7. Astrology & Religious Festival Calculation** 🔮  
-- Used in **Hindu Panchang, Islamic Calendar, Christian Easter Date** calculations.  
-- Example: Finding **Easter Sunday** for a given year.  
+7. Astrology & Religious Festival Calculation
+- Used in Hindu Panchang, Islamic Calendar, Christian Easter Date calculations.
+- Example: Finding Easter Sunday for a given year.
 
 ---
 
-### **8. Customer Subscription and Membership Renewals** 🎫  
-- Used in **Netflix, Amazon Prime, Gym Memberships**, etc.  
-- Example: Reminding users **before their subscription expires**.  
+8. Customer Subscription and Membership Renewals
+- Used in Netflix, Amazon Prime, Gym Memberships, etc.
+- Example: Reminding users before their subscription expires.
+
+🔹 Example Use Case:
 
-🔹 **Example Use Case:**  
-```java
 import java.util.Calendar;
 import java.util.GregorianCalendar;
 
@@ -145,25 +141,27 @@ public class SubscriptionReminder {
         System.out.println("Send reminder on: " + renewalDate.getTime());
     }
 }
-```
 
 ---
 
-## **📌 Why Modern Applications Prefer `java.time` Over `Calendar`?**
-| Feature | `java.util.Calendar` (Legacy) | `java.time` (Modern) |
-|---------|-------------------------------|----------------------|
-| **Mutability** | Mutable (Can be modified) | Immutable (Thread-safe) |
-| **Thread-Safety** | Not thread-safe | Thread-safe |
-| **Ease of Use** | Complex API | Simple API |
-| **Performance** | Slower due to mutable nature | Faster |
-| **Time Zone Handling** | Complicated | Easier |
+Why Modern Applications Prefer java.time Over Calendar:
+
+| Feature             | java.util.Calendar (Legacy)  | java.time (Modern)      |
+|---------------------|------------------------------|-------------------------|
+| Mutability          | Mutable (can be modified)    | Immutable (thread-safe) |
+| Thread-Safety       | Not thread-safe              | Thread-safe             |
+| Ease of Use         | Complex API                  | Simple API              |
+| Performance         | Slower due to mutable nature | Faster                  |
+| Time Zone Handling  | Complicated                  | Easier                  |
+
 
-🚀 **Recommendation:**  
-If you're working on **new Java projects (Java 8+), prefer using `java.time` (`LocalDate`, `ZonedDateTime`) instead of `Calendar`.**  
+Recommendation:
+If you're working on new Java projects (Java 8+), prefer using `java.time` (`LocalDate`, `ZonedDateTime`)
+instead of `Calendar`.
 
-However, in legacy applications**, `Calendar` is still used in many enterprise systems.
 
 ---
 
-### Conclusion ✅
-The Calendar API is useful for applications requiring **date-time calculations**, such as **scheduling, payroll, banking, inventory management, and reservations.
+Conclusion:
+The Calendar API is useful for applications requiring date-time calculations, such as scheduling, payroll, banking,
+inventory management, and reservations.