How do I increment calendar by 7 days until next month, then year from a starting date in Java?

Question

This problem has been bugging me. I've googled around and found increment in java from current date but I want it starting from a certain date in the past.

For example, let's start with this date in this format:

"1986-09-20"

How can I increment the days so that the example above becomes "1986-09-27" and with one more increment it becomes "1986-10-4" all the way until the end of the year by which it becomes "1987-01-03"

I understand that I can just increment the date using += 7 on the day and if it surpasses say 30 then increment the month but I don't know how I can account for days that are either 30 or 31 or February's leap day.

I've been looking at the documentation for Calendar and notice that it may hold the solution but I don't know how/where to start.

I want to result format to be yyyy-mm-dd NOT the Java date format which is Day Month dd time time-zone yyyy

Answer 1

The other Answers are correct but outdated.

Strings != Date-Times

I want to result format to be yyyy-mm-dd NOT the Java date format which is Day Month dd time time-zone yyyy

You are confusing a date-time value with a String that represents that value. So there is no such thing as a “Java date format”. You can generate a String in any format you desire, or generate one automatically in a localized format.

The input string you desire is a standard ISO 8601 format. The java.time classes use these standard formats by default.

java.time

The java.time framework is built into Java 8 and later. These classes supplant the old troublesome date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat. The Joda-Time team also advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP.

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

LocalDate

The LocalDate class represents a date-only value without time-of-day and without time zone.

LocalDate start = LocalDate.parse( "1986-09-20" );

This class can do math, adding years and weeks as you desire.

Apparently you want to start with the first occurrence of the day-of-week of your input date in the following month. The TemporalAdjuster interface and TemporalAdjusters class (note the plural) provide such assistance. In particular, TemporalAdjusters.nextOrSame( DayOfWeek ) where DayOfWeek is a handy enum for the seven days of the week.

DayOfWeek dow = start.getDayOfWeek();
LocalDate monthLater = start.plusMonths( 1 );  // If day 29-31 not found in following month, the last day of month is used instead. So always next month, not two months out.
LocalDate firstOfNextMonth = monthLater.withDayOfMonth( 1 );
TemporalAdjuster adjuster = TemporalAdjusters.nextOrSame( dow );
LocalDate firstDayOfWeekOfNextMonth = firstOfNextMonth.with( adjuster );

And a year later.

LocalDate yearLaterFromFirstDayOfWeekOfNextMonth = firstDayOfWeekOfNextMonth.plusYears( 1 );

Strings

To generate a String in your desired ISO 8601 standard format, simply call toString().

String output = yearLaterFromFirstDayOfWeekOfNextMonth.toString();

Looping

You can loop date-time values by testing with equals,compareTo, isEqual, isBefore, and isAfter methods found on the various java.time classes.

LocalDate ld = start;
while ( ! ld.isAfter( firstDayOfWeekOfNextMonth ) ) {
    String output = ld.toString();
    …
    // Prepare for next loop.
    ld = ld.plusWeeks( 1 );
}

I suggest determining your boundary dates first, to keep the business logic clear. Then afterwards do the looping week-by-week as that is incidental to the logic. Mixing the two issues (boundary dates & week-by-week) is what makes your Question confusing.

Answer 2

Is the following what you are looking for?

Calendar cal = Calendar.getInstance();
cal.clear();
cal.set(1986, 8, 20);
int year = cal.get(Calendar.YEAR);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
while (year >= cal.get(Calendar.YEAR)) {
    cal.add(Calendar.DAY_OF_MONTH, 7);
    System.out.println(sdf.format(cal.getTime()));
}

the output:

1986-09-27
1986-10-04
1986-10-11
1986-10-18
1986-10-25
1986-11-01
1986-11-08
1986-11-15
1986-11-22
1986-11-29
1986-12-06
1986-12-13
1986-12-20
1986-12-27
1987-01-03

Answer 3

Use Joda Time if you can. It will make everything a lot easier and it deals with leap years etc.

here is an example of how simple it is:

DateTimeFormatter dtf = DateTimeFormat.forPattern("yyyy-MM-dd");
DateTime dateTime = dtf.parseDateTime("1986-09-20");

DateTime result = dateTime.plusDays(7);
System.out.println("Result: " + dtf.print(result));