Get Date based on day count in Java

Question

Simple Question, but Google surprisingly had little on this. I have the number of days from Jan 1st of the year. How can I convert that to a date i

Answer 1

Using GregorianCalendar and simpleDateFormat:

int days = 20;
GregorianCalendar dayCountofYear = new GregorianCalendar(2020,Calendar.JANUARY, 1);
dayCountofYear.add(Calendar.DAY_OF_YEAR, days);
System.out.println("GregorianCalendar: "+ dateFormat.format(dayCountofYear.getTime()));

Above logic can be simplified in a function call as below:

Date nextDate = addDaysToDate("01/01/2020", 20);
System.out.println("After Adding Number of Days:"+nextDate); // O/P:21/01/2020

Full Example:

public class DateHelper {

    public static void main(String[] args) throws ParseException {
        int days = 20;
        String initialDay = "01/01/2020";
        Date nextDate = addDaysToDate(initialDay, days);
        System.out.println("After Adding Number of Days:"+nextDate);

        /*Date firstDate = DateHelper.parseDate(initialDay, formatStr);
        int daysBetween = getDaysBetween(firstDate, nextDate);
        System.out.println(days + " - Days Between - "+daysBetween);*/
    }

    static String formatStr = "dd/MM/yyyy";
    static DateFormat dateFormat = new SimpleDateFormat(formatStr);
    static DateFormat reverseDateFormat = new SimpleDateFormat("yyyy/MM/dd");

    public static Date addDaysToDate(String specificDate, int daysCount) throws ParseException {
        Date date = dateFormat.parse(specificDate);
        GregorianCalendar gregCal = (GregorianCalendar) GregorianCalendar.getInstance(); // Current Date
        gregCal.setTime(date); // Change the Date to Provided One
        gregCal.add(Calendar.DAY_OF_YEAR, daysCount);
        Date time = gregCal.getTime();

        ZonedDateTime zdt = gregCal.toZonedDateTime();
        System.out.println("GregorianCalendar DayofWeek: "+ zdt.getDayOfWeek());
        return time;
    }
    public static String addDaysToDateStr(String specificDate, int daysCount) throws ParseException {
        String format = dateFormat.format( addDaysToDate(specificDate, daysCount) );
        System.out.println("GregorianCalendar [To a Date added Number of days leads to New Date]: "+ format);
        return format;
    }
    public static Date parseDate(String aSource, String formatStr) throws ParseException {
        DateFormat dateFormat = new SimpleDateFormat(formatStr);
        dateFormat.setLenient(false);
        return dateFormat.parse(aSource);
    }
    public static String formatDate(Date aDate, String formatStr, Calendar calendar) {
        DateFormat dateFormat = new SimpleDateFormat(formatStr);
        dateFormat.setLenient(false);
        dateFormat.setCalendar(calendar);
        return dateFormat.format(aDate);
    }
    public static String formatDate(Date aDate, String formatStr) {
        DateFormat dateFormat = new SimpleDateFormat( formatStr );
        //dateFormat.setCalendar(Calendar.getInstance(TimeZone.getTimeZone("UTF")));
        return dateFormat.format(aDate);
    }
}