feat(datetime-api): add DateTimeFormatDemo showcasing custom date-time formatting

What
- Introduced DateTimeFormatDemo under DateandTimeAPI package.
- Demonstrates use of DateTimeFormatter with LocalDateTime and ZonedDateTime.
- Added multiple formatting patterns:
  - "dd/MM/yyyy" → formats date only (day/month/year).
  - "dd/MM/yyyy hh:mm:ss" → adds time in 12-hour format.
  - "dd/MM/yyyy hh:mm:ss z" → includes time zone abbreviation using ZonedDateTime.

Why
- Provides clear examples of formatting temporal objects using patterns.
- Shows how different symbols (y, M, d, h, H, m, s, S, z, Z) control output formatting.
- Reinforces distinction between LocalDateTime (no timezone) and ZonedDateTime (timezone aware).

How to use
- Run DateTimeFormatDemo.
- Output will display:
  1. Current system date in dd/MM/yyyy.
  2. Current system date + time in dd/MM/yyyy hh:mm:ss.
  3. Current zoned date-time including timezone (e.g., IST, PST).

Real-life applications
- Formatting timestamps in logging frameworks.
- Displaying human-readable dates in UIs, reports, or invoices.
- Parsing/printing ISO or custom date formats in APIs.
- Handling time zones for global applications.

Notes
- `hh` prints 12-hour format; use `HH` for 24-hour.
- `z` gives short timezone name (e.g., IST), `Z` gives numeric offset (e.g., +0530).
- Thread-safe: DateTimeFormatter is immutable and safe for concurrent use.

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

Author: Someshdiwan

Section26DateandTimeAPI/src/DateandTimeAPI/LocalDateTimeDemo.java

@@ -2,34 +2,39 @@
 import java.time.*;
 import java.time.MonthDay;
 
-/*
-Explanation of Date-Time Classes Mentioned:
-
-ZonedDateTime ? Represents date and time with time zone.
-OffsetDateTime ? Date-time with an offset from UTC.
-OffsetTime ? Time with an offset from UTC.
-ZoneId ? Represents time zone ID.
-MonthDay ? Represents a month and day (e.g., birthday).
-YearMonth ? Represents a year and month.
-Year ? Represents only a year.
-Period ? Represents a period (e.g., 2 years, 3 months).
-Duration ? Represents a duration (e.g., 5 hours, 30 minutes).
-Instant ? Represents a timestamp (date-time in UTC).
-*/
-
 public class LocalDateTimeDemo {
     public static void main(String[] args) {
+        // Current date-time with time zone information (e.g., Asia/Kolkata, UTC offset)
         ZonedDateTime zonedDateTime = ZonedDateTime.now();
+
+        // Current date-time with only UTC offset (e.g., +05:30) but without time zone rules
         OffsetDateTime offsetDateTime = OffsetDateTime.now();
+
+        // Current time of day with an offset from UTC (time only, no date)
         OffsetTime offsetTime = OffsetTime.now();
+
+        // The system’s default time zone (e.g., Asia/Kolkata, America/New_York)
         ZoneId zoneId = ZoneId.systemDefault();
+
+        // Represents a month and day without a year (useful for recurring events like birthdays)
         MonthDay monthDay = MonthDay.now();
+
+        // Represents a combination of year and month (useful for credit card expiry, payroll, etc.)
         YearMonth yearMonth = YearMonth.now();
+
+        // Represents only the year (useful for historical data, age calculation, etc.)
         Year year = Year.now();
-        Period period = Period.of(2, 3, 5); // 2 years, 3 months, 5 days
+
+        // Represents a date-based amount of time (e.g., 2 years, 3 months, 5 days)
+        Period period = Period.of(2, 3, 5);
+
+        // Represents a time-based amount of time (e.g., 5 hours, 30 seconds)
         Duration duration = Duration.ofHours(5);
+
+        // A point in time on the UTC timeline (timestamp)
         Instant instant = Instant.now();
 
+        // Print results
         System.out.println("ZonedDateTime: " + zonedDateTime);
         System.out.println("OffsetDateTime: " + offsetDateTime);
         System.out.println("OffsetTime: " + offsetTime);
@@ -42,3 +47,16 @@ public static void main(String[] args) {
         System.out.println("Instant: " + instant);
     }
 }
+
+/*
+1. ZonedDateTime → Full date-time with a time zone (rules + offset).
+2. OffsetDateTime → Date-time with only a fixed UTC offset (no daylight saving rules).
+3. OffsetTime → Time of day + offset from UTC (no date).
+4. ZoneId → Represents a time zone identifier (like "Asia/Kolkata").
+5. MonthDay → Stores month + day, without year (e.g., 12-25 → Christmas).
+6. YearMonth → Stores year + month (useful for card expiry).
+7. Year → Stores only year value.
+8. Period → Human-readable period (years, months, days).
+9. Duration → Machine-readable time span (hours, minutes, seconds, nanoseconds).
+10. Instant → Machine timestamp, precise point on UTC timeline.
+*/