feat(datetime-api): add DateTimeDemo showcasing legacy java.util.Date usage

What
- Introduced `DateTimeDemo` class to demonstrate various ways of working with `java.util.Date`.
- Covers:
  - `new Date()` → fetches current system date and time.
  - `new Date(System.currentTimeMillis())` → constructs a Date from epoch-based timestamp.
  - `new Date("1/1/1990")` → deprecated string constructor (parses into a Date object).
  - `getTime()` → retrieves milliseconds since epoch (Jan 1, 1970).
  - `getDay()`, `getMonth()`, and `getDate()` → extracts components of the date (day of week, month index, and day of month).

Why
- Provides hands-on examples of the old `java.util.Date` API.
- Useful for understanding how legacy codebases manage dates.
- Highlights deprecated constructors and methods, motivating migration to `java.time` API.

How to use
- Run `DateTimeDemo` to print:
  - Current date/time.
  - Custom epoch-based date.
  - Parsed historical date.
  - Millisecond timestamp.
  - Day, month, and date values.

Real-life applications
- Reading/modifying legacy code where `java.util.Date` is still in use.
- Debugging date-related issues in pre-Java 8 applications.
- Interfacing with old libraries or databases that return `Date` objects.

Notes
- Many `java.util.Date` constructors and methods (e.g., `Date(String)`, `getDay()`, `getMonth()`) are deprecated.
- Prefer modern `java.time` classes (`LocalDate`, `LocalDateTime`, `ZonedDateTime`) for new projects.

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

Author: Someshdiwan