Generate random date of birth

I'm trying to generate a random date of birth for people in my database using a Java program. How would I do this?

import java.util.GregorianCalendar;

public class RandomDateOfBirth {

    public static void main(String[] args) {
	
	    GregorianCalendar gc = new GregorianCalendar();

	    int year = randBetween(1900, 2010);

	    gc.set(gc.YEAR, year);

        int dayOfYear = randBetween(1, gc.getActualMaximum(gc.DAY_OF_YEAR));

        gc.set(gc.DAY_OF_YEAR, dayOfYear);

        System.out.println(gc.get(gc.YEAR) + "-" + (gc.get(gc.MONTH) + 1) + "-" + gc.get(gc.DAY_OF_MONTH));

    }

    public static int randBetween(int start, int end) {
        return start + (int)Math.round(Math.random() * (end - start));
    }
}

java.util.Date has a constructor that accepts milliseconds since The Epoch, and java.util.Random has a method that can give you a random number of milliseconds. You'll want to set a range for the random value depending on the range of DOBs that you want, but those should do it.

Very roughly:

Random  rnd;
Date    dt;
long    ms;

// Get a new random instance, seeded from the clock
rnd = new Random();

// Get an Epoch value roughly between 1940 and 2010
// -946771200000L = January 1, 1940
// Add up to 70 years to it (using modulus on the next long)
ms = -946771200000L + (Math.abs(rnd.nextLong()) % (70L * 365 * 24 * 60 * 60 * 1000));

// Construct a date
dt = new Date(ms);

Snippet for a Java 8 based solution:

Random random = new Random();
int minDay = (int) LocalDate.of(1900, 1, 1).toEpochDay();
int maxDay = (int) LocalDate.of(2015, 1, 1).toEpochDay();
long randomDay = minDay + random.nextInt(maxDay - minDay);

LocalDate randomBirthDate = LocalDate.ofEpochDay(randomDay);

System.out.println(randomBirthDate);

Note: This generates a random date between 1Jan1900 (inclusive) and 1Jan2015 (exclusive).

Note: It is based on epoch days, i.e. days relative to 1Jan1970 (EPOCH) - positive meaning after EPOCH, negative meaning before EPOCH

---

You can also create a small utility class:

public class RandomDate {
    private final LocalDate minDate;
    private final LocalDate maxDate;
    private final Random random;

    public RandomDate(LocalDate minDate, LocalDate maxDate) {
        this.minDate = minDate;
        this.maxDate = maxDate;
        this.random = new Random();
    }

    public LocalDate nextDate() {
        int minDay = (int) minDate.toEpochDay();
        int maxDay = (int) maxDate.toEpochDay();
        long randomDay = minDay + random.nextInt(maxDay - minDay);
        return LocalDate.ofEpochDay(randomDay);
    }

    @Override
    public String toString() {
        return "RandomDate{" +
                "maxDate=" + maxDate +
                ", minDate=" + minDate +
                '}';
    }
}

and use it like this:

RandomDate rd = new RandomDate(LocalDate.of(1900, 1, 1), LocalDate.of(2010, 1, 1));
System.out.println(rd.nextDate());
System.out.println(rd.nextDate()); // birthdays ad infinitum

You need to define a random date, right?

A simple way of doing that is to generate a new Date object, using a long (time in milliseconds since 1st January, 1970) and substract a random long:

new Date(Math.abs(System.currentTimeMillis() - RandomUtils.nextLong()));

(RandomUtils is taken from Apache Commons Lang).

Of course, this is far to be a real random date (for example you will not get date before 1970), but I think it will be enough for your needs.

Otherwise, you can create your own date by using Calendar class:

int year = // generate a year between 1900 and 2010;
int dayOfYear = // generate a number between 1 and 365 (or 366 if you need to handle leap year);
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.YEAR, randomYear);
calendar.set(Calendar.DAY_OF_YEAR, dayOfYear);
Date randomDoB = calendar.getTime();

For Java8 -> Assumming the data of birth must be before current day:

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.Period;
import java.time.temporal.ChronoUnit;
import java.util.Random;

public class RandomDate {

	public static LocalDate randomBirthday() {
		return LocalDate.now().minus(Period.ofDays((new Random().nextInt(365 * 70))));
	}
	
	public static void main(String[] args) {
		System.out.println("randomDate: " + randomBirthday());
	}
}

If you don't mind adding a new library to your code you can use MockNeat (disclaimer: I am one of the authors).

MockNeat mock = MockNeat.threadLocal();

// Generates a random date between [1970-1-1, NOW)
LocalDate localDate = mock.localDates().val();
System.out.println(localDate);

// Generates a random date in the past
// but beore 1987-1-30
LocalDate min = LocalDate.of(1987, 1, 30);
LocalDate past = mock.localDates().past(min).val();
System.out.println(past);

LocalDate max = LocalDate.of(2020, 1, 1);
LocalDate future = mock.localDates().future(max).val();
System.out.println(future);

// Generates a random date between 1989-1-1 and 1993-1-1
LocalDate start = LocalDate.of(1989, 1, 1);
LocalDate stop = LocalDate.of(1993, 1, 1);
LocalDate between = mock.localDates().between(start, stop).val();
System.out.println(between);

Generating random Date of Births:

import java.util.Calendar;

public class Main {
  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
        System.out.println(randomDOB());
    }
  }

  public static String randomDOB() {

    int yyyy = random(1900, 2013);
    int mm = random(1, 12);
    int dd = 0; // will set it later depending on year and month

    switch(mm) {
      case 2:
        if (isLeapYear(yyyy)) {
          dd = random(1, 29);
        } else {
          dd = random(1, 28);
        }
        break;

      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        dd = random(1, 31);
        break;

      default:
        dd = random(1, 30);
      break;
    }

    String year = Integer.toString(yyyy);
    String month = Integer.toString(mm);
    String day = Integer.toString(dd);

    if (mm < 10) {
        month = "0" + mm;
    }

    if (dd < 10) {
        day = "0" + dd;
    }

    return day + '/' + month + '/' + year;
  }

  public static int random(int lowerBound, int upperBound) {
    return (lowerBound + (int) Math.round(Math.random()
            * (upperBound - lowerBound)));
  }

  public static boolean isLeapYear(int year) {
    Calendar calendar = Calendar.getInstance();
    calendar.set(Calendar.YEAR, year);
    int noOfDays = calendar.getActualMaximum(Calendar.DAY_OF_YEAR);

    if (noOfDays > 365) {
        return true;
    }

    return false;
  }
}

You can checkout randomizer for random data generation.This library helps to create random data from given Model class.Checkout below example code.

public class Person {

	@DateValue( from = "01 Jan 1990",to = "31 Dec 2002" , customFormat = "dd MMM yyyy")
    String dateOfBirth;

}
    
//Generate random 100 Person(Model Class) object 
Generator<Person> generator = new Generator<>(Person.class);  
List<Person> persons = generator.generate(100);                          

As there are many built in data generator is accessible using annotation,You also can build custom data generator.I suggest you to go through documentation provided on library page.

Look this method:

public static Date dateRandom(int initialYear, int lastYear) {
	if (initialYear > lastYear) {
		int year = lastYear;
		lastYear = initialYear;
		initialYear = year;
	}
	
	Calendar cInitialYear = Calendar.getInstance();
	cInitialYear.set(Calendar.YEAR, 2015);
	long offset = cInitialYear.getTimeInMillis();
	
	Calendar cLastYear = Calendar.getInstance();
	cLastYear.set(Calendar.YEAR, 2016);
	long end = cLastYear.getTimeInMillis();
	
	long diff = end - offset + 1;
	Timestamp timestamp = new Timestamp(offset + (long) (Math.random() * diff));
	return new Date(timestamp.getTime());
}

I think this will do the trick:

public static void main(String[] args) {
    Date now = new Date();
    long sixMonthsAgo = (now.getTime() - 15552000000l);
    long today = now.getTime();

    for(int i=0; i<10; i++) {
        long ms = ThreadLocalRandom.current().nextLong(sixMonthsAgo, today);

        Date date = new Date(ms);

        System.out.println(date.toString());
    }

}

If you don't mind a 3rd party library, the Utils library has a RandomDateUtils that generates random java.util.Dates and all the dates, times, instants, and durations from Java 8's date and time API

LocalDate birthDate = RandomDateUtils.randomPastLocalDate();
LocalDate today = LocalDate.now();
LocalDate under18YearsOld = RandomDateUtils.randomLocalDate(today.minus(18, YEARS), today);
LocalDate over18YearsOld = RandomDateUtils.randomLocalDateBefore(today.minus(18, YEARS));

It is in the Maven Central Repository at:

<dependency>
  <groupId>com.github.rkumsher</groupId>
  <artifactId>utils</artifactId>
  <version>1.3</version>
</dependency>

simplest method:

public static LocalDate randomDateOfBirth() {
    final int maxAge = 100 * 12 * 31;
    return LocalDate.now().minusDays(new Random().nextInt(maxAge));
}

Using the original answer and adapting it to the new java.time.* api and adding ways to generate n random dates -- the function will return a List.

// RandomBirthday.java
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class RandomBirthday {

    public static List<String> getRandomBirthday(int groupSize, int minYear, int maxYear) {
        /** Given a group size, this method will return `n` random birthday
         * between 1922-2022 where `n=groupSize`.
         * 
         * @param   groupSize   the number of random birthday to return
         * @param   minYear     the min year [lower bound]
         * @param   maxYear     the max year [upper bound]
         * @return              a list of random birthday with format YYYY-MM-DD
         */
        
        ArrayList<String> birthdays = new ArrayList<>();
        DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("yyyy-MM-dd");

        for (int i = 0; i < groupSize; i++) {
            LocalDate baseDate = LocalDate.now();
            LocalDate baseYear = baseDate.withYear(ThreadLocalRandom.current().nextInt(minYear, maxYear));

            int dayOfYear = ThreadLocalRandom.current().nextInt(1, baseYear.lengthOfYear());

            LocalDate baseRandBirthday = baseYear.withDayOfYear(dayOfYear);

            LocalDate randDate = LocalDate.of(
                baseRandBirthday.getYear(),
                baseRandBirthday.getMonth(),
                baseRandBirthday.getDayOfMonth()
            );

            
            String formattedDate = dateFormat.format(randDate);
            birthdays.add(formattedDate);
        }

        return birthdays;
    }

    public static void main(String[] args) {
        // main method
        List<String> bDay = getRandomBirthday(40, 1960, 2022);
        System.out.println(bDay);
    }
}

I am studying Scala and ended up Googling Java solutions for choosing a random date between range. I found this post super helpful and this is my final solution. Hope it can help future Scala and Java programmers.

import java.sql.Timestamp

def date_rand(ts_start_str:String = "2012-01-01 00:00:00", ts_end_str:String = "2015-01-01 00:00:00"): String = {
	val ts_start = Timestamp.valueOf(ts_start_str).getTime()
	val ts_end = Timestamp.valueOf(ts_end_str).getTime()
	val diff = ts_end - ts_start
	println(diff)
 	val ts_rand = new Timestamp(ts_start + (Random.nextFloat() * diff).toLong)
 	return ts_rand.toString
}                                         //> date_rand: (ts_start_str: String, ts_end_str: String)String

println(date_rand())                      //> 94694400000
                                              //| 2012-10-28 18:21:13.216

println(date_rand("2001-01-01 00:00:00", "2001-01-01 00:00:00"))
                                              //> 0
                                              //| 2001-01-01 00:00:00.0
println(date_rand("2001-01-01 00:00:00", "2010-01-01 00:00:00"))
                                              //> 283996800000
                                              //| 2008-02-16 23:15:48.864                    //> 2013-12-21 08:32:16.384

int num = 0;
char[] a={'a','b','c','d','e','f'};
String error = null;
try {
    num = Integer.parseInt(request.getParameter("num"));
    Random r = new Random();
    long currentDate = new Date().getTime();
    ArrayList<Student> list = new ArrayList<>();
    for (int i = 0; i < num; i++) {
        String name = "";
        for (int j = 0; j < 6; j++) {
            name += a[r.nextInt(5)];
        }

        list.add(new Student(i + 1, name, r.nextBoolean(), new Date(Math.abs(r.nextLong() % currentDate))));
    }

    request.setAttribute("list", list);
    request.setAttribute("num", num);
    request.getRequestDispatcher("student.jsp").forward(request, response);
} catch (NumberFormatException e) {
    error = "Please enter interger number";
    request.setAttribute("error", error);
    request.getRequestDispatcher("student.jsp").forward(request, response);
}