[Solved] How can I add one month to change into the milliseconds?

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I suggest: First, instead of your variables (fields?) year, month, day, hours and minutes just declare

private LocalDate date;
private LocalTime time;
private long milliseconds;

(Keep the milliseconds variable since you will want to have your result here.)

In onDateSet assign a value to date in this way:

        date = LocalDate.of(selectedYear, selectedMonth + 1, selectedDate);

This is where we are taking into account, as you said, that the date picker’s selectedMonth is 0-based, while LocalDate numbers months the way humans do, from 1.

Now your timePicker method becomes

private void timePicker(){
    // Get Current Time
    time = LocalTime.now(ZoneId.systemDefault());
    // Launch Time Picker Dialog
    TimePickerDialog timePickerDialog = new TimePickerDialog(this,
            android.R.style.Theme_Holo_Light_Dialog,
            new TimePickerDialog.OnTimeSetListener() {

                @Override
                public void onTimeSet(TimePicker view, int hourOfDay,int minute) {

                    time = LocalTime.of(hourOfDay, minute);

                    milliseconds = date.atTime(time)
                            .atZone(ZoneId.systemDefault())
                            .toInstant()
                            .toEpochMilli();
                    Log.e("LONGGGGGGG", String.valueOf(milliseconds));
                }
            },
            time.getHour(),
            time.getMinute(),
            true);
    timePickerDialog.show();

}

You were greatly overcomplicating things. I recommend:

  1. Don’t use strings for dates or times in your business logic, use date and time objects. This also means you’ve got no need for a formatter like SimpleDateFormat (which, I might add, is notoriously troublesome, so it is good that you can do without it).
  2. Use java.time, the modern Java date and time API. It is so much nicer to work with than the old-fashioned date and time classes like Calendar.

The two points go hand in hand in the above snippets. Also as far as I can see you have no use for the Calendar object (c) in setDateField, just remove it and all use of it.

What went wrong in your code?

The culprit was your SimpleDateFormat. Try using it like this:

    SimpleDateFormat f = new SimpleDateFormat("yyyy/mm/dd HH:mm");
    System.out.println(f.parse("2018/04/17 12:45"));

On my computer this prints

Wed Jan 17 12:45:00 CET 2018

It prints a date in January no matter what is in the string. This is because you tried lowercase mm for month. mm is for minutes (which you also used correctly), month is uppercase MM. So your formatter cannot parse a month at all and assigns your date the default month, which is the first month of the year, January. And this date in January was of course also what went into your millesecond value.

Question: Can I use java.time on Android?

Yes, you can use java.time on Android. Using it just requires Java 6.

  • In Java 8 and later and newer Android versions the new API comes built-in.
  • In Java 6 and 7 get the ThreeTen Backport, the backport of the modern classes (ThreeTen for JSR 310, where the modern API was first described).
  • On older Android, use the Android edition of ThreeTen Backport. It’s called ThreeTenABP. Make sure you import the date and time classes from package org.threeten.bp and subpackages.

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solved How can I add one month to change into the milliseconds?