When i modified the selected date of the CalendarView I got a negative month and 31+ days in month

Question

i try to get a previous date of custom date that selected by a user but i cant find a way to do that this is the code

 calendar.setOnDateChangeListener(new CalendarView.OnDateChangeListener() {
            @Override
            public void onSelectedDayChange( @NonNull CalendarView view, int year, int month, int dayOfMonth ) {
                finalDate = (dayOfMonth + 7) + "/" + (month - 3) + "/" + year;

                try {
                    Date date = new SimpleDateFormat("dd/MM/yyyy").parse(finalDate);


                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        });

so if i select 30/2/2020 the result is: (37/-1/2020)

Answer 1

java.time and ThreeTenABP

Consider using java.time, the modern Java date and time API, for your date work. Not least when you need to math on dates. I frankly find it much better suited than the old and outdated Calendar class, not to mention Date and SimpleDateFormat .

    int year = 2020;
    int month = Calendar.MARCH; // For demonstration only, don’t use Calendar in your code
    int dayOfMonth = 30;
    
    LocalDate selectedDate = LocalDate.of(year, month + 1, dayOfMonth);
    LocalDate finalDate = selectedDate.minusMonths(3).plusDays(7);
    
    System.out.println(finalDate);

Output:

2020-01-06

I believe that your Android date picker numbers months from 0 for January through 11 for December, so we need to add 1 to convert to the natural way that humans and LocalDate number months. When we start out from 30th March, we subtract 3 months and get 30th December, then add 7 days and get 6th January. We might also have done the math in the opposite order:

    LocalDate finalDate = selectedDate.plusDays(7).minusMonths(3);

In this case it gave the same result, but since months have unequal lengths, it won't always.

Answer 2

Isn't it because you are adding 7 to your day count and subtracting 3 from your month count? Try removing that and it should work better.