Java Date and Time: How do I make plus() and until

Question

Most of the time, these 2 methods are each others inverse:

• Temporal Temporal.plus(long, TemporalUnit)
• long Temporal.until(Temporal, TemporalUnit)

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For example starting from 1-JAN:

System.out.println("1-JAN plus 1 month: " +
    LocalDate.of(2017, 1, 1).plus(1, ChronoUnit.MONTHS));
System.out.println("1-JAN until 1-FEB in months: " +
    LocalDate.of(2017, 1, 1).until(LocalDate.of(2017, 2, 1), ChronoUnit.MONTHS));

They are each others inverse:

1-JAN plus 1 month: 2017-02-01
1-JAN until 1-FEB in months: 1 // GOOD

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However, in this example starting from 31-JAN:

System.out.println("31-JAN plus 1 month: " +
    LocalDate.of(2017, 1, 31).plus(1, ChronoUnit.MONTHS));
System.out.println("31-JAN until 28-FEB in months: " +
    LocalDate.of(2017, 1, 31).until(LocalDate.of(2017, 2, 28), ChronoUnit.MONTHS));

They are not each others inverse:

31-JAN plus 1 month: 2017-02-28 // Bad? Should ceil to 1 March?
31-JAN until 28-FEB in months: 0 // Or is this bad? Should be 1?

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How can I make them to be each others inverse?

Answer 1

There is only 28 days between those dates so the number of whole months is 0. However you could use a bit of rounding if you want less than 1 month be be considered a month. e.g.

// compare the months after rounding to the next month
LocalDate a = LocalDate.of(2017, 1, 31);
LocalDate b = LocalDate.of(2017, 2, 28);
long months = a.plus(1, ChronoUnit.MONTHS)
        .until(b.plus(1, ChronoUnit.MONTHS), ChronoUnit.MONTHS);
System.out.println(a + " until " + b + " in months: " + months);

prints

2017-01-31 until 2017-02-28 in months: 1

and

// compare the months after rounding to the next month
LocalDate a = LocalDate.of(2017, 1, 31);
LocalDate b = LocalDate.of(2017, 3, 30);
long months = a.plus(1, ChronoUnit.MONTHS)
        .until(b.plus(1, ChronoUnit.MONTHS), ChronoUnit.MONTHS);
System.out.println(a + " until " + b + " in months: " + months);

prints

2017-01-31 until 2017-03-30 in months: 2

I probably want the plus to ceil instead of to floor

You might want to treat all months as notionally 30 days depending on your requirements.