A demo code to calculate user's year, date and time of birth

datetime.java

package com.root.datetime;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.Instant;
import java.time.LocalDate;
import java.time.Period;
import java.time.ZoneId;
import java.util.Date;
import java.util.Scanner;

/**
 * 
 * @author Samuel Nwankwo
 * @category Date and Time
 * @problem How to calculate age?
 *
 */
public class CalculateAge {

	public static void main(String[] args) {

		Scanner input = null;
		try {
			input = new Scanner(System.in);
			System.out.println("Enter your DOB in dd/MM/yyyy format :");
			String dob = input.next();
			Date current = new Date();
			// If Date is valid, converting String to date.
			Date mydob = getValidDate(dob);
			if (mydob != null && mydob.before(current)) {

				// Get default Time zone Id.
				ZoneId defaultZoneId = ZoneId.systemDefault();

				// Convert Date mydob to LocalDate
				Instant instant1 = mydob.toInstant();
				LocalDate localeDateMyDob = instant1.atZone(defaultZoneId).toLocalDate();
				// Convert Date current to LocalDate
				Instant instant2 = current.toInstant();
				LocalDate localeDateCurrent = instant2.atZone(defaultZoneId).toLocalDate();

				Period period = Period.between(localeDateMyDob, localeDateCurrent);
				System.out.println("Age : " + period.getYears() + " Years " + period.getMonths() + " Months " + period.getDays() + " Days");

			} else {
				System.out.println("DOB is invalid.");
			}
		} finally {
			if (input != null) {
				input.close();
			}
		}

	}

	private static Date getValidDate(String date) {

		Date mydate = null;
		if (isValidDateFormat(date)) {
			/**
			 * d -> Day of month 
			 * M -> Month of year 
			 * y -> Year
			 */
			DateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
			/**
			 * By default setLenient() is true. We should make it false for
			 * strict date validations.
			 * 
			 * If setLenient() is true - It accepts all dates. If setLenient()
			 * is false - It accepts only valid dates.
			 */
			dateFormat.setLenient(false);
			try {
				mydate = dateFormat.parse(date);
			} catch (ParseException e) {
				mydate = null;
			}
		}
		return mydate;
	}

	private static boolean isValidDateFormat(String date) {

		/**
		 * Regular Expression that matches String with format dd/MM/yyyy. 
		 * dd -> 01-31 
		 * MM -> 01-12 
		 * yyyy -> 4 digit number
		 */
		String pattern = "(0?[1-9]|[12][0-9]|3[01])\\/(0?[1-9]|1[0-2])\\/([0-9]{4})";
		boolean result = false;
		if (date.matches(pattern)) {
			result = true;
		}
		return result;
	}

}