feat(datetime-api): add DateDemo2 demonstrating epoch time and Long.MAX_VALUE

What
- Added DateDemo2 class to demonstrate working with epoch-based time in Java.
- Shows:
  - System.currentTimeMillis() → milliseconds since 1st Jan 1970 (Unix epoch).
  - Long.MAX_VALUE → maximum value storable in a long, useful for understanding limits of time representation.

Why
- Introduces the concept of epoch time, the foundation for most date/time APIs and system clocks.
- Helps developers understand how Java internally represents dates and times.
- Provides context on range limitations when storing time values in long.

How to use
- Run DateDemo2.
- Output will display:
  1. Current epoch time in milliseconds.
  2. Long.MAX_VALUE, showing the upper bound of long type.

Real-life applications
- Epoch time is widely used in:
  - Logging frameworks (timestamps).
  - Databases (storing created_at/updated_at fields).
  - Distributed systems (event ordering, synchronization).
  - Scheduling jobs relative to current system time.
- Long.MAX_VALUE awareness helps when working with time calculations, avoiding overflow errors.

Notes
- Epoch time in Java is always measured from 1st Jan 1970 UTC.
- System.currentTimeMillis() is affected by system clock changes; for monotonic time use System.nanoTime().
- Long.MAX_VALUE corresponds to approximately 292 million years in milliseconds → practically “infinite” for real-world use.

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

Author: Someshdiwan

Section26DateandTimeAPI/src/DateDemo.java renamed to Section26DateandTimeAPI/Date/src/DateDemo2.java

@@ -1,7 +1,6 @@
-public class DateDemo {
+public class DateDemo2 {
     public static void main(String[] args) {
-        //1st January 1970 se Ab tak till now.
         System.out.println(System.currentTimeMillis());
         System.out.println(Long.MAX_VALUE);
     }
-}
+}