Count of calendar months touched by span of time in Java

Question

For a span of time running from one date to another date, how to get the number of calendar months containing one or more days of my span?

So for example:

• 2016-01-23/2016-01-23 = 1 calendar month (January)
• 2016-01-31/2016-02-01 = 2 calendar months (January, February)
• 2016-01-23/2016-02-28 = 2 calendar months (January, February)
• 2016-01-15/2016-03-15 = 3 calendar months (January, February, March)
• 2016-01-15/2017-03-15 = 15 calendar months (Jan-Dec of 2016 plus January, February, March of 2017)

I do not define a month as “30 days”. I am asking about calendar months, January-December.

Similar to this Question but that asks about PHP/MySQL.

Answer 1

Calculate the "epoch" month of both dates, then subtract them and add 1.

Using LocalDate like in the other answer, an epochMonth() helper method makes it easy:

private static int monthsTouched(LocalDate fromDate, LocalDate toDate) {
    return epochMonth(toDate) - epochMonth(fromDate) + 1;
}
private static int epochMonth(LocalDate date) {
    return date.getYear() * 12 + date.getMonthValue();
}

Like the results in the question, both dates are inclusive.

Note: Validation skipped for brevity, e.g. what is result if fromDate > toDate?

Test

public static void main(String[] args) {
    test("2016-01-23", "2016-01-23");
    test("2016-01-31", "2016-02-01");
    test("2016-01-23", "2016-02-28");
    test("2016-01-15", "2016-03-15");
    test("2016-01-15", "2017-03-15");
}
private static void test(String fromDate, String toDate) {
    System.out.println(monthsTouched(LocalDate.parse(fromDate), LocalDate.parse(toDate)));
}

Output (matches results from question)

1
2
2
3
15

Answer 2

Use ChronoField.PROLEPTIC_MONTH, which returns a count of months from year zero:

import static java.time.temporal.ChronoField.PROLEPTIC_MONTH;

long monthsTouched = date2.getLong(PROLEPTIC_MONTH) - date1.getLong(PROLEPTIC_MONTH) + 1;

Answer 3

Adjust the start and end dates

The key is to adjust your dates.

• Move the starting date to the first of the month
• Move the ending date to the first of the following month

We move the ending to the next month after because the Half-Open approach is commonly used when considering spans of time. Half-Open means the beginning is inclusive while the ending is exclusive. So lunch hour runs from 12:00 to 13:00 but does not include the 61st minute of 1 PM. A week runs from Monday to Monday, for seven days not including that second Monday.

So a span running from the first of January to the first of March is two months rather than three because we run up to, but do not include, that last date, the first of March.

java.time

The java.time classes built into Java 8 and later make easier work of this.

For date-only values, without a time-of-day and without a time zone, use the LocalDate class.

LocalDate start = LocalDate.parse( "2016-01-31" );
LocalDate stop = LocalDate.parse( "2016-02-01" );

To adjust, use a TemporalAdjuster. Implementations can be found in the TemporalAdjusters class (note the plural 's'). We need firstDayOfMonth and firstDayOfNextMonth.

LocalDate startAdjusted = start.with( TemporalAdjusters.firstDayOfMonth() );
LocalDate stopAdjusted = stop.with( TemporalAdjusters.firstDayOfNextMonth() );

Now use the ChronoUnit class to calculate elapsed whole months.

long calendarMonthsTouched = ChronoUnit.MONTHS.between( startAdjusted , stopAdjusted );

span: 2016-01-31/2016-02-01

calendarMonthsTouched: 2

See this code live in IdeOne.com.